Bridge hunter and John Weeks are good sites to find a bridge that is the length you want. This one was the right length to span the aisle in my trainroom, single track, and no lattice work (1980's bridge). Wandering up the Ohio and Missouri on Google Earth works well too. This particular BNSF bridge in Sioux City IA has a very detailed view on Google Earth, and the overall horizontal dimensions wre taken from the ruler tool on that site.