I'm hoping someone with Kato N-Scale bridges on their layout can give me some dimensions. I'm looking for the height from either the top of the rails, or the bottom of the roadbed to the underside of the deck. I find myself in a bit of a bind in one section of my layout under construction where the clearance is tight. I have clearance with just the track/roadbed passing over some lower tracks but I'm wondering if there still will be clearance if I add a bridge, either a truss or a deck girder.
Clearance above the top of the rails for the Kato single track truss is 43mm, for the double track truss 46mm. Top of rail to underside of bridge is 10.4/10.0mm (single/double), 19.0/19.2mm (single/double) from top of rail to bottom of click-in support beam at both ends. The measurements should be good to +/- 0.1mm.
A truss bridge would give you more clearance than a girder bridge. A minimum of 2 inches is recommender (approx 26'8"). from top of rail to bottom of bridge.
From past experience, if you maintain at least 2 inches you should be fine. If your running autoracks or Amtrak double-deckers you'll need the two inches.
Probably you already considered it but can you lower the track underneath? A pony deck bridge usually provides the most clearance underneath. https://modelerdepot.com/scale-n-pony-truss-steel-bridge-kit-5-1-2-14cm-red
I can't really lower the track underneath, it's on the base of the table, and yeah, something like what you're linking is what I'm going to do, I think. I'm using Kato track though, so I thought the Kato Plate Girder bridge would provide the shallowest profile, from the top of the rail on top to the underside of the deck. That's the dimension I'm looking for. Stefan above provided the numbers for the truss. Just from pictures it looks like the plate girder bridge is less. If the track ends up inside a hill, no problem, I can place the track on a flat plate, maybe a piece of 1/8" hardboard or polystyrene but if it's out in the open, I'll need a bridge. These "engineering" problems are actually kind of fun to solve. On my other O gauge layout something similar happened and I ended up shaving a quarter of an inch from the 1/2" plywood roadbed above from underneath while the track above was still in place. 25 years later, looking at it, I can't even remember how I managed to do it.
I can say for a certainty that the Kato Viaduct pieces have the same measurement from bottom to rail as the truss bridge. I had a truss bridge and it was a PITA to clean, so I swapped it out for the viaduct piece, and it dropped right in... and even looked better too!
Took some pictures - Hope this helps @Xrayvizhen - All the best Above Double Truss Bridge clearance between lower track top of rails to the bottom of bridge 1"5/8" Enough for an Autorack Above Girder Bridge - same Set Up this time clearance top of rail to the bridge is 1" 10/16" - Enough for an Autorack Above SingleTruss Bridge clearance between lower track top of rails to the bottom of bridge 1"5/8" Enough for an Autorack
I am so confused... If there is NOT enough room for cars to clear ANY bridge from underneath...my mind says simply raise the bridge...no matter what kind of bridge it is. Maybe my mind just isnt seeing the problem. If the problems is the height inside a truss bridge...especially Unitrack truss bridges. If you have something that wont clear the inside demensions my mind again says...the problem lies with the load...not the bridge...JMO
I think he can't change his track levels (rail height) above and below, and is looking for the thinnest bridge.
The question is will he ever run anything higher than an autorack. If the load is too high, as @mtntrainman said to adjust the load. I just checked the Double Stack Gunderson Trailer train from Kato still makes it under truss with a bit of room to spare. Most certainly, the Girder has no issue!
A: Easier said than done.This is a small layout with not a lot of room to relocate track around. B. Trying to limit maximum grade to no more than 2%. If I raise the bridge there won't be enough room for the track to loop around and get under other track unless the decline is precipitous. See the beginning of this thread for track plan. Area of the problem is at the bottom right. I think what I'm gonna do is fabricate a bridge in the fashion that Mike Fifer describes in one of his videos using Micro Engineering girders and a piece of something really thin for the deck. I was just wondering about the Kato deck girder bridges since my LHS didn't have one in stock that I could look at.
Thank you Xrayvizhen -- I'll have to see if I can find that link. I'll have a spot on my Unitrack N Scale railroad where a portion of a No. 6 switch will have to be on a bridge. I'm not sure how I'll handle it and your tip might be helpful. I took the photo below in October 1980 at Bethlehem, PA and it's an example of the creative engineering employed by railroads. This was once a normal straight bridge connecting the RDG and CNJ, but in 1965 when the LV and CNJ rationalized their routes, a connection was needed over the Lehigh River between the two roads and this was the result.
It's my unfamiliarity with N scale that's the crux of the problem. Before I set out designing the track plan I googled N scale vertical track clearances and the general consensus was 2 inches. So that's the number I used for weaving the tracks over and under each other. That turned out OK for sections of the track that will be concealed but for the track that's out in the open, I should have allowed an extra 1/2" or maybe a little more for the thickness of a bridge deck. I didn't, so that's why I'm looking at what my options are because at this point I'm not going to pick up the track and start over. If I fabricate a bridge, and it turns out it will need to be a curved bridge) using the Fifer method, things should work out. I'm not worried about autoracks or double decker passenger cars since in the era I'll be modeling, the early 1960's, (May, 1961 if you want to get more specific) I don't think they existed.
For my TTX 5-unit car with two containers on top I measure 42mm from the bottom of the track ties/sleepers to the top of the top container (some container combinations measure less; I hope there are none that measure more...). Subtract 3mm for the total track height (bottom of tie to top of rail), and you need 39mm height for zero (ok, maybe 0.2mm) clearance. Add 10.4mm, and you are at 49.4mm, with a whole 1.4mm clearance to your 50.8mm (2") vertical track spacing. So, you are ok. If you are not sure you are building to those tolerances, then maybe add 1/16th" of elevation to your top track as you approach the bridge. I did of course measure all this out of the goodness of my heart, and not because I am building a layout with the exact same 2" vertical track distance... Seriously: good thing you brought this up, X-ray, and made me drag out the container units; I had assumed 2" was plenty, when in reality it's actually pretty tight.
Is that a gantlet (or gauntlet) track bridge, or are their switches on both ends, and a single track between them? A gantlet has no moving parts, and needs far less maintenance than a couple of switches (though the signaling needs are similar.) But there is no interchange on a gantlet.
