BigJimThe point that I was trying to make was that "a significant amount" is normal for light engine moves as that is the only way that they have to control speed downhill. A "significant amount" is not "abuse". Abuse that would indeed lead to damage.
When I said 'significant amount' what I actually meant was "enough braking effort to heat the tires severely" and this actually involves two things: the severity of the braking effort and the time it is applied. Where the problem develops is certainly not in sustained light-engine braking, even up to what looks like considerable speed; it is when you put the energy of braking into the tire enough to get it to expand 'dangerously' relative to the cooler center, but not enough energy to start warming the wheelrim. (In other words, beginning to approximate what the gas ring does when changing tires intentionally.) Service braking on light engines going down sustained grades is not likely to produce the effect, certainly not when supervised by intelligent N&W engine crews (this is a redundant phrase).
There is a bit of overlap between care to keep the tires tight and care to retain tread profile and extend tire life. ATSF 3751, for example, sometimes was moved with a set of spine flatcars in tow; this provided a relatively large number of braked wheels and shoes at comparatively low tare weight, so the independent could be sparingly used even if the engine had to be operated relatively fast to keep traffic on the line it was operating over fluid. I was more of the opinion that the practice was to preserve the tire diameter and profile and eliminate the potential for driver flatting (as notoriously produced on UP 844 during the early Dickens years) through reduced reliance on independent-brake actuation.
Big Jim is correct in that I was largely discussing the ability of the locomotive to brake its own weight, whether or not the locomotive is actually pulling a train. The GG1 problem was interesting because it was a combination of two sources that were recognized but not understood together: the braking effort required from the older rolling stock on G-hauled clockers was considerable, even at the lower permissible general running speed, as could be noted from the severe heating and smoking observable at, say, 30th Street. I made the mistake then of thinking this was due to poor contemporary car maintenance, when it actually represented part of decelerating the locomotive weight. When Amfleet was substituted, even at the 'legal' 100mph top speed, the combination of regulated peak braking effort from the consist and the heavy locomotive weight at the front resulted in severe tire heating, which had not been observed before; at the time this was attributed to running up to 110mph but I have not yet been able to find any evidence of orders allowing that speed in general service above what was permitted by timetable (as Tim Zukas requested). (The energy dissipated from service braking from 110mph would be over 20% greater than that from 100mph for an equal-time deceleration). In any case there is no question that the change led to thrown tires (and the threat of more!) but this is far outside what I'd expect to see in most operations with steam.
Balt,All true.The point that I was trying to make was that "a significant amount" is normal for light engine moves as that is the only way that they have to control speed downhill. A "significant amount" is not "abuse". Abuse that would indeed lead to damage.
The problem is that those who don't know read this and believe that the least little bit of heat is going to throw a tire. Worse, they repeat it until it becomes a myth.
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BigJim Overmod One of the principal reasons blended air and independent braking was restricted on large steam locomotives is that any significant amount of driver braking to slow the mass of the locomotive can result in the tires expanding enough to work off the center. Various kinds of clips, Gibson rings, etc. were used in an attempt to preclude the issue. The problem was that the light Amfleet equipment couldn't supply "its share" of braking effort to help decelerate the 250+-ton locomotive, which threw a somewhat disproportionate share of the high-speed braking on the (non-Decelostat-equipped) G's driver brakes. I keep reading of this and to some point it is true and makes sense. However, in this day and age I think that many are putting too much emphasis on this heat thing when using the independent brake. If things were as bad as many tend to report, how then did light engine movements get over the road without losing its tires? Think of all of those pusher engines backing down lite from Blue Ridge to Boaz! Reverse moves were restricted to 25 mph, so, the engine brakes will need to be used for a good distance (BTW, I have sound recordings of pushers going well over 25 mph downhill). Those tender brakes aren't going to control the speed all by themselves. Pushers returning from Lofton to Roanoke on the Shenandoah line dealt with even steeper grades than the puny Blue Ridge. And, they had the advantage of being able to turn around on wye tracks in order to return engine first. Throwing a tire was not a problem. As I was told, those brake shoes were made to absorb the heat.
Overmod One of the principal reasons blended air and independent braking was restricted on large steam locomotives is that any significant amount of driver braking to slow the mass of the locomotive can result in the tires expanding enough to work off the center. Various kinds of clips, Gibson rings, etc. were used in an attempt to preclude the issue. The problem was that the light Amfleet equipment couldn't supply "its share" of braking effort to help decelerate the 250+-ton locomotive, which threw a somewhat disproportionate share of the high-speed braking on the (non-Decelostat-equipped) G's driver brakes.
The problem was that the light Amfleet equipment couldn't supply "its share" of braking effort to help decelerate the 250+-ton locomotive, which threw a somewhat disproportionate share of the high-speed braking on the (non-Decelostat-equipped) G's driver brakes.
I keep reading of this and to some point it is true and makes sense. However, in this day and age I think that many are putting too much emphasis on this heat thing when using the independent brake. If things were as bad as many tend to report, how then did light engine movements get over the road without losing its tires? Think of all of those pusher engines backing down lite from Blue Ridge to Boaz! Reverse moves were restricted to 25 mph, so, the engine brakes will need to be used for a good distance (BTW, I have sound recordings of pushers going well over 25 mph downhill). Those tender brakes aren't going to control the speed all by themselves. Pushers returning from Lofton to Roanoke on the Shenandoah line dealt with even steeper grades than the puny Blue Ridge. And, they had the advantage of being able to turn around on wye tracks in order to return engine first. Throwing a tire was not a problem. As I was told, those brake shoes were made to absorb the heat.
The independent on light engine moves is not attempting to hold in check thousands of tons of train as they are when braking a train, even when train brakes are applied.
I suspect that when handling light engines that the brakes are applied with less pressure and for a shorter amount of time than when handling a full sized train. The less tonnage being retarded, the more effective a given brake pipe reduction will feel, and the quicker speed will be held in check.
Drivers and tires can absorb a lot of heat, however there is a limit. The tire being the friction surface in braking will recieve a lot more heat in braking operation than will the wheel itself and the wheel is a bigger heat sink than is the tire. Tires will absorb a grater percentage of heat than will brake shoes, however brake shoes may reach higher temperatures than tires when brakes are applied.
Never too old to have a happy childhood!
OvermodOne of the principal reasons blended air and independent braking was restricted on large steam locomotives is that any significant amount of driver braking to slow the mass of the locomotive can result in the tires expanding enough to work off the center. Various kinds of clips, Gibson rings, etc. were used in an attempt to preclude the issue. The problem was that the light Amfleet equipment couldn't supply "its share" of braking effort to help decelerate the 250+-ton locomotive, which threw a somewhat disproportionate share of the high-speed braking on the (non-Decelostat-equipped) G's driver brakes.
Needless to say, the original BART cars were thyristor control with DC series motors - somewhere in my collection of papers there is a couple of handouts with the schematics of the propulsion electronics and a brochure on the Westinghouse traction motors. This was at a power systems seminar at UCB back when I was an undergrad in Cal's EECS department.
If my memory isn't too far gone, the motors were rated at 150 to 160 hp at 550 volts, and two motors were permannetly connected in series. What impressed me was that the thyristor control was sophisticated enough to operate the motors as series generators. Acceleration was 3mph/sec up to 30mph and the cars were initailly rated for an 80mph top speed which was cut back to 70mph to lessen wear and tear on the motors. In the early days of BART operations, the trains would get up to 80MPH for a few seconds when running the 1.1 miles between the Berkeley Ashby station and main Berkeley station - running time would be 1'35".
A couple of memories, the ~310Hz hum from the smoothing reactors coupled with whirring of the motor-alternator providng the AC power for the A/C and lights.
If I were to design a clean sheet propulsion system for BART (keeping the 1kV third rail potential), the system would use 1700V SiC FET modules for inverter per motor. The FET's would allow for a high enough switching frequency to allow for small and relatively inexpensive filter.This would allow the use of ordinary wire for winding the motors.
Some years ago I was travelling on the underground tracks in Sydney, Australia on a 1980s "Tangara" train, which was noted for its modern styling. For example the upper deck windows curved into the roof much like Superliner lounge cars, not a feature expected on commuter trains.
I was riding just behind the cab and the window in the door was not covered by a blind as is usually the case. While I couldn't see the driver, I had a view of the control panel, and in the tunnels I could see the ammeter, which consisted of illuminated LED segments in a circle, giving a display similar to a conventional analog gauge. The segments lit up green for power and red for regeneration. THis was a train with DC motors and Thyristor control.
As we approached a station, power would be cut off and regenerative brakes applied. The regeneration was maintained most of the way down the platform, with the disc brakes applied at a rekatively slow speed. The change was best seen on the ammeter sibnce there was no obvious jerk as the brakes changed from one mode to another.
Peter
The original BART cars from ca 1970 had blended regenerative/dynamic braking and friction braking. The regenartive/dynamics alludes to the braking controller pushing out current at 1kV (BART third rail voltage) with onboard resistors being cut in when the bus voltage went aove ~1100V due to nothing around to absorb power.
Been reading early editions of Railway Electrical Engineer and noted that work on regenerative braking started in the 1890's.
Canpost The UK does not use dynamic breaking. I believe they use the two pipe air brake system. Since they had very few single pipe freight cars the implementation was fairly straight forward.
The UK does not use dynamic breaking. I believe they use the two pipe air brake system. Since they had very few single pipe freight cars the implementation was fairly straight forward.
Many freight cars in the UK only have single-pipe air brakes (some have twin-pipe), but passenger cars always have twin-pipe air brakes.
Older UK diesel locomotives don't have dynamic braking (it's normally called 'rheostatic braking' here), but the more recent GE-built 'Class 70' and Vossloh/Stadler built 'Class 68' locomotives do have it fitted.
Regenerative or rheostatic braking is very common on electric locomotives all over Europe (UK included) and has been for many years.
Modern electric MU passenger trains commonly have electronically controlled, fully 'blended' electric and friction braking, controlled from a single power/brake handle i.e. 'coast' in the centre position, move one direction for power, the other for braking.
SD70Dude Does the Cascade tunnel have enough clearance to allow both catenary and doublestacks?
Does the Cascade tunnel have enough clearance to allow both catenary and doublestacks?
I don't think so, the picture I've seen of a W-1 electric emerging from the tunnel shows the pantograph close to lock down position.
A battery+electric locomotive could make electrfication a much more viable proposition as the wire could be dead in low clearance zones, which would eliminate the need to raise clearances in those areas. The 1991-92 SCRRA hearings on RR electrification in the LA basin suggested that half of the cost of electrification was from increasing clearances such as highway overpasses.
OTOH, building a few battery demonstrator locomotives would be cheaper than all but the smallest electrification projects.
Shadow the Cats owner The first production engines with dynamic brakes on them were the FTs delivered to the Santa Fe in 1939 unit 100 her trial by fire was Cajon pass with a maximum tonnage frieght. They made it down without heating up the wheels at all.
The first production engines with dynamic brakes on them were the FTs delivered to the Santa Fe in 1939 unit 100 her trial by fire was Cajon pass with a maximum tonnage frieght. They made it down without heating up the wheels at all.
Santa Fe FT #100 was delivered in January 1941. The EMC FT demonstrator #103 dates to 1939.
Erik_Mag I'm not really sold on battery locomotives yet, though what may be the best test site for such beasts would be helper engines on the BNSF Cascade tunnel line. This may be a way to increase capacity of that line as the reduction in the number of diesel engines should allow for shorter wait times between trains.
I'm not really sold on battery locomotives yet, though what may be the best test site for such beasts would be helper engines on the BNSF Cascade tunnel line. This may be a way to increase capacity of that line as the reduction in the number of diesel engines should allow for shorter wait times between trains.
One could also build shorter sections of catenary, to boost range and allow battery locomotives to charge "on the fly".
CP in Rogers Pass would be another good test site, and the Mt. MacDonald tunnel was designed to allow for future catenary installation.
Greetings from Alberta
-an Articulate Malcontent
I've know a couple of people who worked at Maxwell a long while back, so had been familiar with their "ultracap" technology, but was amazed by stumbling on their modules when looking up large value caps in Digi-Key. Price was reasonable for the project being worked on at the time and as a lark, decided to calculate cycle life energy costs (price/(energy capacity x number of cycles), the result was impressive. Only problem was that in order to get 500,000 cycles in ten years (expected calendar life), the caps would need to be cycled 6 times per hour for 24hrs/day). Flat switching fits the bill...
Ultracaps appear to be better suited for taking the shocks inherent with locomotive applications than lead acid batteries used on the first green goats. It probably would be a good idea to give them some shock protection and some form of environmental control to keep capacitor internal temperatures to under 60C. It would pay to be generous with energy storage capacity as less of the power would be used up by internal resistance, which nicely translates into less internal heating and longer cycle life.
One key selling point for this locomotive is instantaneous response, no waiting for the engine to load (or a genset to come on-line). A related note was that the Milwaukee actually strung wires on industrial spurs in the Butte/Deer Lodge area beacuse the electric switchers were much more responsive than the diesel switchers.
Erik_MagOTOH, I think a switcher with bank of ultra-capacitors would make sense...
It does, and I'd add that many of the approaches used for KERS would apply (as they do in wayside storage for hump applications etc.) This is particularly true for much of the current flat-switching emphasis where very high currents are involved for durations probably less than a minute, with very little transition between full charge and full discharge rate (all this being NOT conducive to long chemical-battery life)
BTW the translation of those figures into hp/hr is approximately 0.375 to 0.75 -- then correct using the efficiency of what will likely be inverter AC drive.
Slow throttling up and down is an important reduction of emissions; I would add that slow throttle-up combined with transient reduction of alternator field will reduce nanoscale PM even more dramatically.
CSSHEGEWISCH Not quite, diesels can't have regenerative braking since they have no way of returning current to the catenary or third rail.
Not quite, diesels can't have regenerative braking since they have no way of returning current to the catenary or third rail.
They can if they have a battery on-board, as in the case of GE's proposed hybrid diesel-electric locomotive.
OTOH, I think a switcher with bank of ultra-capacitors would make sense, the ultra-caps would recover energy from braking and also provide instant response for acceleration. Cost of energy storage based on cycle lifetime is abour $0.05/kwhr for the capacitor modules, and probably $0.10/kwhr when the auxiliaries are included. An additional benefit is that the throttling of the prime mover can be done at a much slower rate, which should improve emissions.
NDG At some point ' Regenerative Braking ' was applied to Diesel Electric Locomotives. Thank You.
BaltACD 10-4.....thanks for the info. kenny dorham Ah, OK.....after my time. Not even aware of that category. I wrenched on a Formula Ford for 2 seasons in 1978 and 1979. We were leading our first race when my driver crashed in (what was then) turn-9 at Laguna Seca. Kelly was still just 18 years old at that point, we would not be 19 until September.! I never got to Elkhart Lake. But all the pics i have ever seen look beautiful. I went to Road Atlanta twice for The Nationals...1979 and 1980. I towed a car back for an MGB guy...maybe that was C-Production back then.? I was walking around the track one night (Atlanta) and i ran into two older guys that turned out to be Buddy Baker and Paul Newman. I was able to hang-out with them, Just The Three Of Us, for about 20 minutes that night. They both seemed like decent guys. So anyway.......that is why i asked. Formula 500 originated in 1979 as Formula 440 - The class uses snowmobile engines and CVT drive trains. The class was redesignated at Formula 500 in 1996 when 500cc engines were permitted, both the 440cc and 500cc engines were 2-strokes. in 2009 a class with 4-stroke 600cc motorcycle engines and transmissions was created as F600. In 2014 that class was 'forced' into F500 with restrictor plates being required. In the 2015 Runoff's at Daytona, one of the MC engine cars turned at trap speed of 159.868 MPH, the fastest 2-strokes topped out at about 145 MPH. There has been a 'war' over what size restrictors the MC engine need to run for the sake of equal competition. I last raced at Road Atlanta in 2005 - which I think is the last year Paul Newman competed in GT-1. Saw him in the paddock and on track but never hat the opportunity to talk with him.
10-4.....thanks for the info.
kenny dorham Ah, OK.....after my time. Not even aware of that category. I wrenched on a Formula Ford for 2 seasons in 1978 and 1979. We were leading our first race when my driver crashed in (what was then) turn-9 at Laguna Seca. Kelly was still just 18 years old at that point, we would not be 19 until September.! I never got to Elkhart Lake. But all the pics i have ever seen look beautiful. I went to Road Atlanta twice for The Nationals...1979 and 1980. I towed a car back for an MGB guy...maybe that was C-Production back then.? I was walking around the track one night (Atlanta) and i ran into two older guys that turned out to be Buddy Baker and Paul Newman. I was able to hang-out with them, Just The Three Of Us, for about 20 minutes that night. They both seemed like decent guys. So anyway.......that is why i asked.
I wrenched on a Formula Ford for 2 seasons in 1978 and 1979. We were leading our first race when my driver crashed in (what was then) turn-9 at Laguna Seca. Kelly was still just 18 years old at that point, we would not be 19 until September.!
I never got to Elkhart Lake. But all the pics i have ever seen look beautiful. I went to Road Atlanta twice for The Nationals...1979 and 1980. I towed a car back for an MGB guy...maybe that was C-Production back then.?
I was walking around the track one night (Atlanta) and i ran into two older guys that turned out to be Buddy Baker and Paul Newman. I was able to hang-out with them, Just The Three Of Us, for about 20 minutes that night. They both seemed like decent guys.
So anyway.......that is why i asked.
Formula 500 originated in 1979 as Formula 440 - The class uses snowmobile engines and CVT drive trains. The class was redesignated at Formula 500 in 1996 when 500cc engines were permitted, both the 440cc and 500cc engines were 2-strokes. in 2009 a class with 4-stroke 600cc motorcycle engines and transmissions was created as F600. In 2014 that class was 'forced' into F500 with restrictor plates being required. In the 2015 Runoff's at Daytona, one of the MC engine cars turned at trap speed of 159.868 MPH, the fastest 2-strokes topped out at about 145 MPH. There has been a 'war' over what size restrictors the MC engine need to run for the sake of equal competition.
I last raced at Road Atlanta in 2005 - which I think is the last year Paul Newman competed in GT-1. Saw him in the paddock and on track but never hat the opportunity to talk with him.
jeffhergertThere is at least one photograph out there of a steam engine that threw off a driver tire while underway.
There's at least one photograph out there of a GG1 that threw off a driver tire while underway.
One of the principal reasons blended air and independent braking was restricted on large steam locomotives is that any significant amount of driver braking to slow the mass of the locomotive can result in the tires expanding enough to work off the center. Various kinds of clips, Gibson rings, etc. were used in an attempt to preclude the issue.
The GG1 problem came about when the 'second era' of Gs substituting on Metroliner schedules came about in the late 1970s. The problem was that the light Amfleet equipment couldn't supply "its share" of braking effort to help decelerate the 250+-ton locomotive, which threw a somewhat disproportionate share of the high-speed braking on the (non-Decelostat-equipped) G's driver brakes. Didn't take long for the effects to become notorious -- and that was probably after a certain amount of tire walking and separation in service. (Something to consider: once the tire has expanded even a slight degree, it's no longer in contact with the effective heat sink of the driver center over most of its rotation, which enhances the relative isolation of brake-induced heating (during the time the tire is clamped to the center by brakeshoe force) in the metal of the tire...). I suspect that helping to address this may be part of the effect of German system of crossed brakeshoes/levers used for driver braking on some of the faster locomotives.
kenny dorhamAh, OK.....after my time. Not even aware of that category. I wrenched on a Formula Ford for 2 seasons in 1978 and 1979. We were leading our first race when my driver crashed in (what was then) turn-9 at Laguna Seca. Kelly was still just 18 years old at that point, we would not be 19 until September.! I never got to Elkhart Lake. But all the pics i have ever seen look beautiful. I went to Road Atlanta twice for The Nationals...1979 and 1980. I towed a car back for an MGB guy...maybe that was C-Production back then.? I was walking around the track one night (Atlanta) and i ran into two older guys that turned out to be Buddy Baker and Paul Newman. I was able to hang-out with them, Just The Three Of Us, for about 20 minutes that night. They both seemed like decent guys. So anyway.......that is why i asked.
kenny dorham Wow...thanks to Both Of You. I certainly did not realize the complexity of the braking system. I have seen Youtube Videos of workers in the repair shops, where they would ignite a Ring Of Fire around the tire for a locomotive and then just hammer it onto the wheel i guess.? So yeah, it would make sense that a certain amount of heat, while train is moving, might be a bad idea for the Loco Tire. BTW...BaltACD, are those Formula Fords in your Avatar.?
Wow...thanks to Both Of You. I certainly did not realize the complexity of the braking system.
I have seen Youtube Videos of workers in the repair shops, where they would ignite a Ring Of Fire around the tire for a locomotive and then just hammer it onto the wheel i guess.? So yeah, it would make sense that a certain amount of heat, while train is moving, might be a bad idea for the Loco Tire.
BTW...BaltACD, are those Formula Fords in your Avatar.?
There is at least one photograph out there of a steam engine that threw off a driver tire while underway. Because of the side rods, the tire didn't fly off into the ditch but would've sort of flailed around until they stopped. Which the crew was able to do before the train was derailed.
Jeff
Ah, OK.....after my time. Not even aware of that category.
kenny dorhamBTW...BaltACD, are those Formula Fords in your Avatar.?
No they are Formula 500's. My son is in the red car passing me going through the Carousel at Road America outside Elkhart Lake, WI as we competed in the Sports Car Club of America National Champinship Runoff's back in 2010. My son is fast, I am only half fast.
kenny dorham I have seen Youtube Videos of workers in the repair shops, where they would ignite a Ring Of Fire around the tire for a locomotive and then just hammer it onto the wheel i guess.? So yeah, it would make sense that a certain amount of heat, while train is moving, might be a bad idea for the Loco Tire.
The tire would be heated, causing it to expand slightly. It would then be slipped over the wheel where it would shrink to a tight fit as it cooled.
BRC used tires on its diesels until relatively recently.
The railroad world of steam and the railroad world of today are as different as night and day. In freight service steam was not able to move the size of the trains that exist today. Car sizes and capacities were smaller in the steam era. In the steam era a 50 foot box car was a giant. The majority of the cars had a load capacity of 50 tons or less. Where sustained braking was required over a several mile mountain grade - retainers were used to keep train brakes applied on the cars that had their retainers activated. With driver tires being shrunk fit on driver wheels, sustained engine braking was not an option - heating the driver tires could cause them to expand and come off their whees (a uncommon but potential occurence).
Digging through a FRA Accident Report of a runaway that happened on CSX. FRA testing revealed on the 2% grade involved car brakes would fade to ineffectiveness when subjected to speeds over 15 MPH with 100 ton loads, which present a total car weight of 130-145 tons per car. In the particular incident the train had 3 units all dynamic brake equipped, however DB wasn't functioning on the rear two units. TTSI stated that if a train was exceeding 15 MPH at a particular point that the train was to be brought to a stop. The train was doing about 18 MPH at that point with a application on the train and DB at max amps indicated and the crew then place the train brake in emergency - and the train continued to gain speed until it finally derailed.
The FRA then instructed the carrier not to operate trains in excess of 15 MPH on the grades in that territory.
Getting todays trains up a grade is a relative piece of cake; getting todays trains SAFELY down the grades is where the Engineers earn their pay.
Here is a first-hand account of retainer operation, from my area. This train still runs today (using modern diesels with dynamic braking) but retainers are still used while loading the train.
http://caboosecoffee.blogspot.com/2011/11/air-brakes-on-alberta-coal-branch.html
I've actually gotten to work that train a few times, and CN crews are still expected to set retainers before train loading starts (fortunately not while the train is moving!).
Without dynamics and pressure-maintaining locomotive brake valves train crews were required to set retainers in order to safely descend long, steep grades. Sometimes trains also had to make stops to allow the wheels to cool before proceeding.
A retainer is a valve found on the exhaust pipe from a car's brake cylinder. It's normal position (Direct Exhaust) allows the air brake to release normally, but the retainer can also be set to different positions which make the car take longer to release (Slow Direct), or not release completely (High Pressure). Older cars had a "Low Pressure" position too, which kept a lighter brake application than "High Pressure".
Pressure-maintaining is a feature of the locomotive's automatic brake valve which adds just enough air to the train's brake pipe to compensate for leaks, but not enough to trigger a release.
To understand the rationale behind these features one must learn a bit more about how the air brake system works. Once the system is charged one reduces the brake pipe pressure to set the brakes, and any futher leakage will result in a heavier brake application. When descending a long, even grade without a pressure-maintaining brake valve this leakage will eventually result in the brake application becoming heavy enough that the locomotive(s) cannot pull the train downhill anymore (even in full throttle), so the Engineer must release the brake in order to continue.
A car's air brake (except for some passenger equipment) can only be released completely, the Engineer cannot back off a bit to a lighter application. It only takes a 1 or 2 PSI rise in brake pipe pressure to trigger a release. The brakes will then release fairly quickly, long before the system has fully recharged. Now the Engineer must make another application to control the train, but will have to make a larger reduction in brake pipe pressure in order to achieve the same braking effort, as the system is not fully recharged yet. Doing this is called Cycle Braking, and doing it too many times will result in not enough air left to control the train. The technical term for this is "Peeing away the Air", and it has caused many runaways over the years.
This is where the retainers come in. Setting them to "Slow Direct" allows the train to recharge for a longer time before the brakes release, and setting them to "High Pressure" prevents them from releasing completely. Setting enough retainers to either position gives the Engineer more control.
Though they are rarely used anymore every car is still required to have a retainer. Today they are normally found at a lower level somewhere around the side of the car, but historically they were placed high up on the end of the car, right next to the stemwinder handbrake. This placement allowed both handbrake and retainer to be operated while the train was moving, with the Brakemen walking along the tops of cars.
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