How tall does my bridge need to be in OO gauge?

If you are thinking of installing a bridge over your track on a model railway, how high should it be?

You may think the answer to that question is high enough for your trains to fit through! and you would be correct, but there is more to consider.

If you are planning on using ready-to-plant portals, then how high your bridge should be is of little concern as everything is worked out for you, but, if you are planning on making your own or using a kit, a little more thought is required.

The measurement that is likely to cause you to stop and think is how much clearance on top of your train's height should you allow for. Most ready-made portals allow for between 1 and 2cm (half an inch to one inch) above the roof of a regular locomotive or coach. That level of clearance equates to a portal height of roughly 7cm (3in) above the baseboard at its apex and is sufficient for regular rolling stock to pass through. And that should be your first consideration, ask yourself if all of your rolling stock is regular. Cranes and locomotives with pantographs require additional height, so make sure you use your tallest rolling stock as a reference when designing your bridge. If you think you may want to install catenary at a later date, make sure you plan for that too, ripping out a hill or roadway later to make alterations will be soul-destroying!

Another common problem area when designing a bridge is a lack of appreciation for how much the arch of a double-track portal encroaches into the running path of trains. The centre of such a portal will have to be closer to 9cm (just under 4in) above baseboard level at the apex because trains run through off-centre, so test, test and test again before you commit to a final design.