Just trying to see if i'm right. Using the NMRA standards gauge, from the top of the rails I need a 3" clearance for an overpass. Thanks in advance.
Randy
I have read everything from 3 1/2 inches to slightly higher than your highest car. I was challenged for space to keep my grade under the 4% maximum so went with 2 3/4 inches. I believe I read that there was no prototypically standard height of tunnels and overpasses.
Also depends on your era. Back in the '50s, cars weren't generally as tall, TOFC and intermodal was still in its infancy. So you could get away with lower clearances than you would need for double-stacks and auto-racks. Since I do model the 50's, my minimum clearance (on my branchline) will be 16 scale feet, or *about* 2-1/4 inches ... might just go 2-1/2 just to round things more evenly. The main will probably use closer to 3", just in case I want to run an oversize load.
Brad
EMD - Every Model Different
ALCO - Always Leaking Coolant and Oil
CSX - Coal Spilling eXperts
eaglescout I have read everything from 3 1/2 inches to slightly higher than your highest car. I was challenged for space to keep my grade under the 4% maximum so went with 2 3/4 inches. I believe I read that there was no prototypically standard height of tunnels and overpasses.
Prototype "standards" varied between the railroads. Even in the 1930"s and before there were lines that could not handle the tallest cars. The advent of "excess height" cars and double stacks really raised the bar.
The Western Pacific Railroad cut notches into the roof of many of their tunnels to accomodate double stacks in the Feather River Canyon. In Niles Canyon, they lowered the grade of the track.
If you determine the height of tallest equipment you will run you can set the clearance based on that. However, you should have more clearence if possible, in case you want to run taller equipment in in the future, or a an oversize load.
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
Don't forget to measure the caboose smoke jacks!
Dave
I'm just a dude with a bad back having a lot of fun with model trains, and finally building a layout!
As with all planning, consider contingencies! Is your current taste and interest likely to be locked for the duration of your layout's usefulness to you? Is it possible that you might want to have a MOW or wrecker train? If so, you will want more height for the boom. As stated just above, the stacks on cabees can sometimes surprise you. Happened to me with a True Line CPR caboose. A catenary or double-stacks will add to the height requirement. At full extension, a pantograph on a Trix HO GG1 is nearly 4" off the deck!!!
Think ahead.
Crandell
My minimum clearance was determined by the allowable grades, and at just under 2 1/2", won't allow certain equipment to use this wye:
Out on the mainline, the minimum is 2 3/4" and will allow most equipment to pass. However, the loaded car shown below exceeds that limit, and is routed to avoid the obstacles:
When these cars are emptied, they're returned to the shipper via the same route, as the semi-permanent rack supporting the load must always be at the south end of the car.
Many industrial sidings have limited clearances, both at the sides and overhead. I deliberately placed the elevated storage bins at GERN Industries so that the clearance under them is very limited. Not only are all locos prohibited from the tracks beneath the bins, but many freight cars are also too high. Crews need to ensure that cars to be spotted for loading meet the height restrictions, but also need to be careful when selecting idler cars to pick-up cars spotted some distance into the 220'-long siding - flats and gondolas work well, but many house cars exceed the 2 1/8" overhead restriction.
If you're regularly running tall equipment, your mainline clearances should reflect that, but don't be afraid to introduce areas elsewhere that have restrictions of some sort, as it adds operational interest.
Wayne
Something to consider is that the upper track doesn't have to be the only track with a grade. You can put the lower track in a depression to achieve more clearance.
Not sure what the NMRA gauge is set for. I have a Mark II gauge, but there are newer ones. The prototype has 'Plates' with similar clearance diagrams like the NMRA gauge. I have 3" from rail to the bottom of a bridge(22 scale ft).. Our club uses the same or better - and they run double stacks, tri-level auto racks, etc with no problems.
The following chart was lifted from Wikipedia(and I suspect they got it from the AAR web site):
As you can see, 20' 2" seems to be the maximum clearance standard for double stacks and auto racks. Many times model manufacturers will make their models ride higher to get more truck swing(and be able to handle sharper curves). This will make out scale model even higher than the prototype. For this reason, our club has a minimum clearance of 22' above the rail.
Jim
Modeling BNSF and Milwaukee Road in SW Wisconsin