96RAF Posted January 18, 2020 Share Posted January 18, 2020 I have to make a bridge over a lower track, but I need to know the minimum distance from top of the bottom board taking account the use of code 100 rails to the underside of the top board to provide adequate clearance for the tallest 00 rolling stock on the market. Board thicknes is 9 mm or 0.3”. Currently I have allowed 0.25” for the track height including sleepers, and I have measured my tallest stock as 2.5” absolute max, so that equates to a 2.75“ plus a whisker inter-board dimension. A notional 3” would work between upper and lower board heights, but I don’t want to have to rise higher than is necessary for the passing crossing, nor to make it look wildley out of scale, nor do I want to build it to lesser limits if a taller rolling stock may foul later on. Any advice appreciated apart from ‘don’t use gradients’. I have plenty of room to build them in and they re essential to the trackplan. Link to comment Share on other sites More sharing options...
walkingthedog Posted January 18, 2020 Share Posted January 18, 2020 Rob I have code 100 laid straight on the board. I have a bridge crossing the track with a clearance of 2 3/4inches. Looks just right and everything passes under it from steam to diesel and coach to crane. Not sure about a loco with a pantograph but I will never own one. Link to comment Share on other sites More sharing options...
LCDR Posted January 18, 2020 Share Posted January 18, 2020 On the 'real' railway the distance from the top of the rail to the underside of a bridge should be 15 feet minimum or in other words 60mm (2.3 inches) in 4mm scale. A locomotive should be a maximum of 13 feet 6 inches to its highest point. As you have already deducted allowing for your track height a clearance of 2.75 inches should be adequate. The only thing that might require you to increase that dimension is if you wanted to put overhead electric wires up. You would then need about 3.5 inches. Link to comment Share on other sites More sharing options...
walkingthedog Posted January 18, 2020 Share Posted January 18, 2020 Wow looks like I got in right. Link to comment Share on other sites More sharing options...
Chrissaf Posted January 18, 2020 Share Posted January 18, 2020 I too wanted to minimise the rise on my gradients (my layout is half up, half down layout design so the track is in essence split over three levels). I settled on a gap of 63mm as being the minimum to cater for all the potential rolling stock I would ever likely to run on my layout. I too have absolutely no intention of ever running anything with a pantograph. Link to comment Share on other sites More sharing options...
96RAF Posted January 18, 2020 Author Share Posted January 18, 2020 Me too Brian - for once.I have a Class 90 and a 395 but with the pantos wired down, so I think 2.75” it will be +/- a tad. No likelihood of me putting up electric string for my electric locos. 63 mm being 2.4” so plenty of slack at 2.75” which is a tad under 70 mm. In Scarm I have the respective track heights at 3” and 6” so exactly right given board thickness and track height. 3” is base board level with 0” the level of the drop valley, purely to get the software to draw properly as it doesn’t do negative heights. Link to comment Share on other sites More sharing options...
walkingthedog Posted January 19, 2020 Share Posted January 19, 2020 I'm a negative 6ft 2in, that's 5ft 10 in. 😆 Link to comment Share on other sites More sharing options...
37lover Posted January 19, 2020 Share Posted January 19, 2020 I am not one of those members that can upload & immediately show photos, or show links, but if you type model railway dimensions into Google you will easily find tables/diagrams showing the recommended clearances for bridges, tunnels, platforms, etc. My eyesight is not great but I think it is showing 60mm as the clearance from top of rail to underside of bridge. Link to comment Share on other sites More sharing options...
atom3624 Posted January 19, 2020 Share Posted January 19, 2020 Apologies if already covered. Fairly recently, the Liverpool - Manchester line has been overhead electrified and bridges raised 'for clearance issues'. Is there a before / after electrification height, or when talking of 1:76.2 the difference is negligible?Just a thought. Al. Link to comment Share on other sites More sharing options...
LCDR Posted January 19, 2020 Share Posted January 19, 2020 Lines electrified on the overhead wire systerm. The normal height of the contact wire is 16 feet above rail level, For 25kv AC the minimum height at overbridges and other low structures is 14 feet 5 inches, and the minimum height at public road level crossings is 18 feet 6 inches. The contact wire is staggered on alternate sides of of the centre line approiximately 8 inches to even out wear on the carbon head of the pantograph. Therefore a 15 foot minimum height bridge will normally be OK but many older bridges were below the minimum which required them to be raised. This was often done by replacing the brick arch with pre-cast concrete sections, or in the case of steel girder bridges these would be jacked up, and the abutments raised and the road reprofiled. Link to comment Share on other sites More sharing options...
walkingthedog Posted January 19, 2020 Share Posted January 19, 2020 In tunnels they lower the track bed I believe. Link to comment Share on other sites More sharing options...
LCDR Posted January 19, 2020 Share Posted January 19, 2020 It really depended on the tunnel design. Some tunnels were the wrong shape to do this and it would be far too expensive. Another technique was to break out some of the brickwork in the top of the tunnel and replace this with a concrete cut out so that the wire suspension arms could be inset to get the necessary clearances. The wire would be only about 8 inches from the tunnel brickwork, but the mechanical suspension and insulators needed more space. Link to comment Share on other sites More sharing options...
walkingthedog Posted January 19, 2020 Share Posted January 19, 2020 Very interesting. Link to comment Share on other sites More sharing options...
LCDR Posted January 19, 2020 Share Posted January 19, 2020 Coincidentally I have only just been looking at a drawing of how they did this. Link to comment Share on other sites More sharing options...
Buz Posted January 20, 2020 Share Posted January 20, 2020 Hi RAF96Add an extra 6" to your grades 3" top 3" bottom to make sure you can get a good transition from flat to slope,and keep playing with it until you get a nice smooth derailment free and uncoupling free transition.Most problems had with grade's are at the transition points from flat to slope or trying to do it in ridiculous silly short length.If you have not got one get a grade level you will need it nothing less than perfect will do.As to bridge height I am pretty sure its 3" or 75mm.I know it is not what you would normally even think of running but the Bachmann 50' High cube box car is useful as a Height clearance tester the 85' High cube (can't remember who makes those) is better still, as you can mark side clearances as well use that and it is unlikely anything you have or might visit will foul anywhere.And just in case the minimum height for a tunnel access hatch is around about 6" 150 mm so you can get hands in to extract the derailed train.regards John Link to comment Share on other sites More sharing options...
96RAF Posted January 20, 2020 Author Share Posted January 20, 2020 Hi John.Thanks for the advice. I am aware of the importance of transitions, having had them on a previous folded 8 layout, which is why I am using 9 mm ply for the top boards as it does not bend as sharp as thinner material. As well as long and short spirit levels I also use one of these to set my slopes - extremely useful. About 2” x 2” x 1” and has a magnetic base. Cost less than £20 over 15 years ago, when I needed a pocket size device to use in the field./media/tinymce_upload/6c87fdefebecc6557a9dce56fd8841fc.jpg I have no tunnels therefore no access problems and I have decided to standardise on a 3” board to board clearance having run a mock up with my talllest stock. Most of my locos and coaches are of much the same height within 1/4”. the only bridge is on the flat so no additional transition effects to allow for. Link to comment Share on other sites More sharing options...
Aussie Fred Posted September 18, 2020 Share Posted September 18, 2020 I know this is a bit late on this subject but just came upon the problem. I have constructed footbridge and loco shed from a different brand (they were family gifts) and when I put them on the layout, they didn't look right - just 50cm clearance. neadless to say, my Mallard knocked the bridge for a six and got stuck trying to get into loco shed. The box clearly said OO/HO and never had this trouble with Hornby stuff. Link to comment Share on other sites More sharing options...
96RAF Posted September 18, 2020 Author Share Posted September 18, 2020 Sounds like they were more Ho than OO Fred. Link to comment Share on other sites More sharing options...
Puffed Out Posted September 18, 2020 Share Posted September 18, 2020 Just about a put girder road bridge over a double track, on a curve. Got the height right, about 60mm from the rail to the the underside of the bridge. 125 HST clears by about 9mm..I have measured a few Locos, and they all seem the same height, which is understandable..Now my question is.!......Is that enough clearance for all 00 scale rolling stock. +/- a couple of mm.? Link to comment Share on other sites More sharing options...
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