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Why does AC traction provide increased adhesion?

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Why does AC traction provide increased adhesion?
Posted by Leo_Ames on Monday, July 5, 2021 1:37 PM

Most of the advantages of AC traction motors are clear cut.

The simpler traction motors that lack features like the brushes and commutators found in DC motors means less maintenance, less potential or perhaps even complete elimination of the risk for flashovers and ground relay faults, etc. Easy for the average railfan such as myself to grasp that this all results in power that's more durable and reliable, cheaper to maintain, spends less time out of service, etc. 

And thanks to the thermal load that they can handle, AC traction eliminates short time ratings. That means that they can work harder for extended periods of time all while visiting the shop less frequently than DC motored power does. Also easy for railfans to wrap their head around.

But what I personally don't understand and have never seen explained, is what perhaps is their biggest advantage over DC traction. Can someone explain to a layman like myself, what it is about AC motors that enables AC traction equipped locomotives to have increased adhesion compared to their DC motored counterparts? 

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Posted by bogie_engineer on Monday, July 5, 2021 1:41 PM

The simple answer is that there is no fear of a runaway motor during a wheelslip; it runs at whatever the controlled speed of the applied frequency creates. So it can be pushed beyond what the DC wheelslip system can control.

Dave

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Posted by Erik_Mag on Monday, July 5, 2021 4:41 PM

To embellish a bit on what Dave wrote: AC induction motors are not quite constant speed, as the speed will drop a bit with increasing load - this is why a 4 pole induction motor is rated at a bit over 1700 rpm instead of the 1800 rpm for a synchronous motor, with high efficiency motors running closer to 1800 rpm than a low efficiency motor. The upshot is that it doesn't take much slip to cause a dramatic reduction in generated torque, which inherently limits the slipping. The adhesion control system acts to fine tune motor control to get the last bit of adhesion from a slow creep form of wheelslip.

A DC series motor needs to be experience a much larger amount of slipping before the torque falls off (this is where the "fear of a ruaway motor" comes from for DC series motors). This means that the adhesion control system must act quickly to prevent a high seed wheelslip, however the inductance of the field and armature windings will limit how fast the wheelslip control can act.

Some of the advantage of an AC motor over the DC series motor can be had by separately exciting the field windings as was done on the orignal AEM-7's.

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Posted by Paul Milenkovic on Monday, July 5, 2021 9:17 PM

bogie_engineer

The simple answer is that there is no fear of a runaway motor during a wheelslip; it runs at whatever the controlled speed of the applied frequency creates. So it can be pushed beyond what the DC wheelslip system can control.

Dave

 

 

Far be it for me to dispute someone "on the ground" during the transition from DC to AC tractions motors, but the AEM-7 electric locomotives derived from Swedish design had a control system for DC motors allowing heretofore unheard levels of tractive effort as a fraction of axle weight.  Whereas the control system for an advanced wheel-slip control is simpler for an AC motor, it is still possible for a DC motor?

Where I see the AC motor as winning out is 1) dispensing with the commutator to transmit electric power to the rotor.  The AC induction motor transfers electric power to the rotor through contact-free electromagnetic induction, hence, induction motor.  The commutator can required maintenance and repair, can "flash over", and had required cutting back on motor current (hence tractive effort) when the wheels are bouncing on crossovers or other trackwork.  2) the rotor of an induction motor replaces many turns of wire that need to be insulated with "shorting bars" or a "squirrel cage" of heavy copper bars embedded in steel.  This means the rotor of an AC motor is rugged mechanically and thermally, allowing a rotor geared for low-speed lugging to not fly apart when run at higher RPMs than the more mechanically and electrically complicated DC motor rotor.  The rotor of an AC motor is much less susceptable to damage from heat at high tractive effort that damages the insulation in a DC motor.  Finally, 3) AC induction motors with variable frequency electronic "drives" are much more energy efficient than DC motors, especially at low speed and high torque, which combined with the simpler design of the AC motor rotor, further gives them the advantage of producing high torque at low rotation speeds in drag service without shortening the life of the motor.

I read that Norfolk Southern was a holdout keeping with DC motors, where for some time DC locomotives were less expensive per locomotive unit, where for some time after the introduction of AC units, you had to run drag freights that lugged locomotives for them to pay, or at least in the opinion of NS management.

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by Paul Milenkovic on Monday, July 5, 2021 9:23 PM

I also thought that AC locomotives had some "weird tricks", such as being able to supply dynamic braking down to standstill or at least very low speed, of being able to balance a train on a grade without friction brakes at a stop without burning up the motors.

Because you no longer worry about a limit on rotor speed, either from the "back EMF" in a DC motor or from the rotor of a DC motor "bird's nesting" at excessive rotational speed, are AC motors even offered in different gear ratios for different types of service?  My understanding is that at least in US freight service, an AC locomotive is "one locomotive serves all" from a drag freight to the "hottest" intermodal train?

If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
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Posted by SD70Dude on Monday, July 5, 2021 9:35 PM

Paul Milenkovic

I also thought that AC locomotives had some "weird tricks", such as being able to supply dynamic braking down to standstill or at least very low speed, of being able to balance a train on a grade without friction brakes at a stop without burning up the motors.

I wouldn't call it a weird trick, but yes, they can do those things.

You have to be careful with AC's, heavy dynamic braking has the potential to cause damage to both the track and the train, in ways that most DC units could only dream of.

Cutting out a traction motor does not disable DB on AC's, while it does on DC's.

Paul Milenkovic

Because you no longer worry about a limit on rotor speed, either from the "back EMF" in a DC motor or from the rotor of a DC motor "bird's nesting" at excessive rotational speed, are AC motors even offered in different gear ratios for different types of service?  My understanding is that at least in US freight service, an AC locomotive is "one locomotive serves all" from a drag freight to the "hottest" intermodal train?

All the AC's I've ever seen are listed as having a 70 mph top speed.  I'm not sure if this is the safe maximum motor rpm, or simply what the overspeed relay is set to trip at.  

I've noticed that DC's seem to pull better than AC's once you get up above about 50 mph, but this is of minor consequence in modern freight railroading, as relatively little time is spent at these high speeds.

Greetings from Alberta

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Posted by Erik_Mag on Monday, July 5, 2021 11:28 PM

SD70Dude

 All the AC's I've ever seen are listed as having a 70 mph top speed.  I'm not sure if this is the safe maximum motor rpm, or simply what the overspeed relay is set to trip at. 

Most like the inverter control is set up to not operate at a frequency higher than what's necessary for 70 mph. Inverter control is very likely driven by computer and it would be very easy to put in a software imposed limit.

 

I've noticed that DC's seem to pull better than AC's once you get up above about 50 mph, but this is of minor consequence in modern freight railroading, as relatively little time is spent at these high speeds.

Interesting. A WAG on my part is that the inverter hits a voltage limit at 50 mph and thus starts delivering less power at higher speeds

Paul:

With a variable speed drive and induction motors, you can get braking effort at zero speed by running the inverter with a reversed phase rotation at a very low speed. At this point the inverter will be feeding power into the motor to be dissipated in the motor windings and squirrel cage. If inverter frequency (and power) is sufficiently increased, the locomotive will start moving backwards.

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Posted by daveklepper on Tuesday, July 6, 2021 5:20 AM

Better wheel-slip control has a minor role in the increased tractive-effort of AC-motored locomotives, both diesel-electric and straight-electric.

The majpr role is what you have already stated:  Larger and longer-period current ratings without fear of heat-induced insullation failure and even flashing.

The amperage limits on DC traction mtors are determined by what the armeture windings can tolerate.  The field-coil windings are not a problem.  The field-coils, which are stationary, can be oversize, but the ameture size is constrained.

The AC traction motors do not have armeture windings, only field-coils, with the rotating armetures replaced by rotating slanted aluminum or copper bars.  The current limit is set only what the field-coils can tolerate.

Higher curent/motor = more tractive effort/motor.

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Posted by SD70Dude on Tuesday, July 6, 2021 6:38 AM

daveklepper

Better wheel-slip control has a minor role in the increased tractive-effort of AC-motored locomotives, both diesel-electric and straight-electric.

All due respect, but as someone who operates the things on a daily basis I have to disagree.  

Even the best DC units will spin out and lose traction before they come close to loading the amperage/tractive effort that AC units can exert.

Greetings from Alberta

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Posted by Overmod on Tuesday, July 6, 2021 10:02 AM

An important thing follows from what Erik said earlier about the 'slip'.  Remember that there is current flowing in that armature, or there would be no magnetic torque on it, and that a reason for the 'slip' is that the rotating magnetic field in the stator windings is inducing that armature current before acting on the field that current generates.

It follows that to get peak rotating torque under 'locked rotor' conditions (e.g. any time the locomotive isn't physically moving) you set the inverter to 'rotate the magnetic field' at the speed corresponding to the slip: this induces max current in the rotor to produce 'starting' torque.

But if an axle were to break loose, its rotor would rapidly break sync with the slowly rotating field and wheeltread adhesion rapidly pull it back into appropriate sync.  I believe modern inverters can adjust the field characteristics to allow the effect of 'microslipping' or creep control comparable to that on DC motors.

What I think Mr. Klepper meant is that no special 'wheelslip control' system is needed on AC inverter-drive locomotives, as they cannot spin to high (essentially voltage-controlled) speed if the coefficient of adhesion sufficiently decreases, or keep turning at a 'balancing speed' that wears crescents in railheads when unnoticed by the 'wrong stuff' in the automatics.

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Posted by bogie_engineer on Tuesday, July 6, 2021 11:28 AM

SD70Dude

All the AC's I've ever seen are listed as having a 70 mph top speed.  I'm not sure if this is the safe maximum motor rpm, or simply what the overspeed relay is set to trip at.  

 

Progress Rail's brochures for the SD70ACe and SD70ACe-T4 list the top speeds as 70 and 75 mph, respectively, although they both use the same inverters and traction motors. I personally have run an SD60MAC at Pueblo up to 96 mph with no concern about bird-nesting although it had to be pushed by a DC loco with higher speed gearing. GE also lists their ET44AC as having a 75 mph top speed. I think with both builders, this is an electrical limit rather than a traction motor durability limit.

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Posted by Overmod on Tuesday, July 6, 2021 1:51 PM

Pretty sure Mr. Goding is right.  The rotor with its bar construction can be reasonably dynamically balanced and I see few thrust issues that might cause it to swell or 'walk' into conflict with the stator poles.  Erik can speak to the speed and current requirements of switching the individual stator windings on and off at the necessary frequency for higher rotational speed (and dealing with the effects of 'back EMF' in adjacent windings from the induced field in the rotor structure).  In my opinion, some of the electrical issues here for high-speed 'high-horsepower' rotation would be common to what would be involved in 'exciting' the rotor during high-speed dynamic braking, if you followed the earlier discussion to a bit of a logical conclusion.

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Posted by SD70Dude on Tuesday, July 6, 2021 2:52 PM

How was the ride quality at 90+ mph?

In addition to the wheel creep control, which works far better than that on DC units, the GE AC units will reduce load to an axle which is slipping excessively (usually the lead one) and redistribute it to the other five.  I'm not sure if EMD units with individual axle control do the same.  

Call it what you want, in layman's terms it all leads back to the same thing: much better wheelslip control.  

Greetings from Alberta

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Posted by BaltACD on Tuesday, July 6, 2021 4:24 PM

AC's can in some cases be referred to as being 'stalled in motion'.

Had a coal train one night report they were about 200 yards from the crest of the grade and the crew, while not stopped, figured it would be a hour or more at their present rate of movement before they crested the grade and would begin to pick up speed.

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Posted by YoHo1975 on Tuesday, July 6, 2021 5:16 PM
Uh, what do you guys mean you've never heard of an AC locomotive with speed over 70MPH? Have you been living under a rock? The Siemens Charger and the EMD F125 both are geared for Max Speed of 125MPH. F125 has EMD A2921-5 Traction motors according to the internet. And wouldn't it be the gearing not the traction motor that determines top speed?
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Posted by SD70Dude on Tuesday, July 6, 2021 5:24 PM

YoHo1975
Uh, what do you guys mean you've never heard of an AC locomotive with speed over 70MPH? Have you been living under a rock? The Siemens Charger and the EMD F125 both are geared for Max Speed of 125MPH. F125 has EMD A2921-5 Traction motors according to the internet. And wouldn't it be the gearing not the traction motor that determines top speed?

I said "all the AC's I've ever seen", meaning in person.  In my case this means the six axle heavy freight locomotives made by GE and EMD (our AC fleet is almost entirely GEVOs)

I work in freight service in western Canada, and as far as I know I've never been within 500 miles of an AC traction passenger locomotive.  

I had always assumed that you would achieve a higher top speed by changing the gearing, just like on DC units.  Looks like there's a bit more to it.

Greetings from Alberta

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Posted by YoHo1975 on Tuesday, July 6, 2021 5:37 PM

In theory, the Chargers should be making call on Vancouver BC via cascades service, but I'm not sure if they are assigned on all routes at this point or if there are still F59s and other locomotives being used. 

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Posted by SD60MAC9500 on Tuesday, July 6, 2021 6:09 PM
 

bogie_engineer

I personally have run an SD60MAC at Pueblo up to 96 mph with no concern about bird-nesting although it had to be pushed by a DC loco with higher speed gearing. 

Not being facetious I'd love to hear more about this. 

 
 
 
Rahhhhhhhhh!!!!
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Posted by bogie_engineer on Tuesday, July 6, 2021 8:46 PM

SD60MAC9500
 

 

 
bogie_engineer

I personally have run an SD60MAC at Pueblo up to 96 mph with no concern about bird-nesting although it had to be pushed by a DC loco with higher speed gearing. 

 

 

Not being facetious I'd love to hear more about this. 

 
 
 
 

During the development of the HTCR truck, we had a requirement to run without hunting instability at a minimum of 75 mph with a worn wheel profile (which we defined as an effective conicity of 1:6 (considers rail and wheel profile jointly)). Testing on EMD3 with the prototype HTCR trucks showed we met that requirement and the truck was released for production on the SD60MAC and SD70. Experience with the HTCR II truck on the SD90/43MAC showed showed truck hunting under the right conditions, worn hollow wheels near condemning on tight gauge track, that really was uncomfortable as it excited a mode of the isolated cab making it hard to stay in the seat. So we had a UP SD90/43MAC at the Pueblo test center (TTCI) to develop a mod to fix it and eventually found tightening up the end axle free lateral was the best solution.

A few years later, about 2001 IIRC, I did another test program at TTCI to explore a few other mods to enhance stability using what loco was available, an SD60MAC, as the test unit. We ran all the tests on the Transit Test Track which has a 100 mph 3" cant deficiency speed based on its curves and superelevation. With this kind of test, you don't learn anything until you take it past its stable speed. Unmodified, it was stable to near 80 mph. The final mod required us to go 96 mph to get truck hunting. None of these tested mods went into production units but they stay in the engineer's "bag of tricks" for when they're needed.

Dave

 

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Posted by Erik_Mag on Tuesday, July 6, 2021 11:38 PM

Overmod

Pretty sure Mr. Goding is right.  The rotor with its bar construction can be reasonably dynamically balanced and I see few thrust issues that might cause it to swell or 'walk' into conflict with the stator poles.

The rotor bars are more mechanically robust than the the windings in a DC motor. A bit like comparing the members of a cantillever bridge with the support cables of a suspension bridge.

 Erik can speak to the speed and current requirements of switching the individual stator windings on and off at the necessary frequency for higher rotational speed (and dealing with the effects of 'back EMF' in adjacent windings from the induced field in the rotor structure).  In my opinion, some of the electrical issues here for high-speed 'high-horsepower' rotation would be common to what would be involved in 'exciting' the rotor during high-speed dynamic braking, if you followed the earlier discussion to a bit of a logical conclusion.

Main issue is that higher speed means higher frequency which then means a higher induced voltage for a given current. Voltage is a combination of current flowing through the various inductances involved with the motor (current times frequency time inductance) along with the magnetic field induced by rotor currents (rotor current times rotor speed). Once the voltage limit of the inverter has been reached, the current fed to the motor will decline as speed increases. One work-around would be to have two sets of stator windings that can be connected in series for low speed operation and in parallel for high speed operation. Other workaround is a higher output volatge for the inverter.

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Posted by oltmannd on Wednesday, July 7, 2021 2:08 PM

This mechanical engineer always had it in his head that some of the "problem" with wheels slip control was how fast you could get the current to change in those big, inductive machines (traction motors and alternators).

Controlling wheelslip on an AC machine means controlling the frequency, which is more or less instantaneous - just lengthen the time from "off" to "on" a smidge.

-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/

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Posted by daveklepper on Wednesday, July 7, 2021 2:25 PM

SD, despite your disagreement, wheel-slip control is not the main reason.  With today's computer technology and with redundancy, inistantneous wheel-slip controkl for DC motor shuld be possible, but why bother?

The simple fact is that for a given suze and weight, one can design an AC non-synchronous induction motor with much greater power, the ability to handle far greater current with the same range of voltage inputs, than a DC (or AC-DC universal) commutator motor.   Period.

You can dissagree all you want to, but you happen to be wrong.  And I do happen to have SB and SM EE degrees from MIT.

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Posted by daveklepper on Wednesday, July 7, 2021 2:30 PM

And I did get to be engineer of an actual revenue freight train om the  Boston  and Maine from Portsmouoth, NH, to Boston (Sommerville Yard) the winter of 1952-1953.  I do not recall any wheel-slip problems on GP-7 1567 that evening.

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Posted by daveklepper on Wednesday, July 7, 2021 2:37 PM

Regarding high speed:  All the currently constructed truly high-speed trains Worldwide use AC non-synchronous induction motors.  All.

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Posted by daveklepper on Wednesday, July 7, 2021 2:55 PM

OK, does a DC commutator motor haver any advantages?  Yes, one:  AC  non-synchronous induction motor locomotives are highly dependent on computer technology.  Redundancy should prevent catastriophic falure.  But if the computer technology is out, the locomtive is out, and the crew cannot put it back into operation.

But an older EMD or Alco DC  locomotive, even with an alternator and rectification, magnetis and/or pneumatic relays formed the logic system for control.  One could see what one was doing.  Of course use insulating gloves!  A really clever crew, with the wiring diagram at hand or in one's head, couild often make temporary repairs and bring the train to its terminal.

The only locomotives I rode as a (student) test engineer on the B&M were EMD and Alco.  I cannot say anything on this topic with regard to the others.

 

 

 

 

 

 

 

 

 

 

 

 

dc-motoed lo

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Posted by BaltACD on Wednesday, July 7, 2021 3:41 PM

daveklepper
OK, does a DC commutator motor haver any advantages?  Yes, one:  AC  non-synchronous induction motor locomotives are highly dependent on computer technology.  Redundancy should prevent catastriophic falure.  But if the computer technology is out, the locomtive is out, and the crew cannot put it back into operation.

But an older EMD or Alco DC  locomotive, even with an alternator and rectification, magnetis and/or pneumatic relays formed the logic system for control.  One could see what one was doing.  Of course use insulating gloves!  A really clever crew, with the wiring diagram at hand or in one's head, couild often make temporary repairs and bring the train to its terminal.

The only locomotives I rode as a (student) test engineer on the B&M were EMD and Alco.  I cannot say anything on this topic with regard to the others. 

dc-motoed lo

Computers - without them the 21st Century world grinds to a halt.  And there is nothing we can do about it, even if we think we can.

Even today's DC traction engines have more computers than you can think of.

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Posted by Backshop on Wednesday, July 7, 2021 7:14 PM

bogie_engineer

The simple answer is that there is no fear of a runaway motor during a wheelslip; it runs at whatever the controlled speed of the applied frequency creates. So it can be pushed beyond what the DC wheelslip system can control.

Dave

 

How refreshing!  An engineer that can explain something to a layman in easy to understand terms and succinctly, at that!  What a rarity around here...

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Posted by Leo_Ames on Wednesday, July 7, 2021 10:09 PM

Thanks for all the helpful responses everyone. Clears up a question I've wondered about for 25 years now. 

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Posted by SD70Dude on Wednesday, July 7, 2021 11:10 PM

daveklepper

SD, despite your disagreement, wheel-slip control is not the main reason.  With today's computer technology and with redundancy, inistantneous wheel-slip controkl for DC motor shuld be possible, but why bother?

NRE was trying to market something like that, I think they called it the SD40-4 with chopper controls or something.  I don't recall if they sold any.  I recall reading that they claimed it gave AC-like performance with DC motors (but probably not the same durability).  

daveklepper

The simple fact is that for a given suze and weight, one can design an AC non-synchronous induction motor with much greater power, the ability to handle far greater current with the same range of voltage inputs, than a DC (or AC-DC universal) commutator motor.   Period.

You can dissagree all you want to, but you happen to be wrong.  And I do happen to have SB and SM EE degrees from MIT.

I'm not denying that AC motors are tougher or can handle higher loads without slipping excessively.

The whole point of going to AC traction is the improved adhesion, i.e. wheelslip control in layman's terms.  The improved adhesion allows the higher power levels to be used effectively, and the tougher motor design allows them to endure that heavier load. 

From my point of view we have a motor (AC induction) fed by a controller (inverter) in a system that is designed to try and reduce or eliminate slipping.  You are controlling wheelslip by minimizing it or preventing it in the first place through the design of the system.   And of course it works very well, not that AC units can't slip, they can and do under certain conditions, and are capable of bucking around quite violently.

AC units are great and are far better than DC's, but they aren't magic.

Also, I might have left out that bragging bit about running the GP7.  I'm sure it was a fun experience and the unit handled the train just fine, but it comes across like me bragging about my electrical knowledge (I can change a lightbulb, and that's about it).  

It has been fascinating to learn some of the details about how the stuff I operate actually works.

Greetings from Alberta

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Posted by daveklepper on Thursday, July 8, 2021 9:29 PM

Bragging?  How many times do "real railroaders" on these Forums put down others?  I simply had to point out that (many yrars ago) I did have at least some "real railroad" experience to go with thw "book learning."  Of course the one trip where I was permitted to be engineer was probably far less important than many trips watching what regolar B&M engineers did, and partucularly how they handled any surprises.

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