oh so here is how you build a bridge, i used masionite and pine for wood
First rip some 3/4 by 3/4 pine into quarters on a table saw if you own one, this will give you some small pieces roughtly 1/4 by 1/4, a bit less but it works
then cut some stips 3/4 by 1/8, then to 1/4 by 1/8, these ar for the braces along the middle, you will need about 16 of these at about 3 inches each
the bottom is masonite, the dimensions i used for a double track bridge are 24" by 5"
i built each section of bracing speratle, you could also but it in 2 pices instead of for like i did for the brige the train is on.
for the bottom of each section you will need some 1/4 by 1/4 at 12", 4 in total
for the tops 1/4 by 1/4 by 8", 4 of these also
now for the verticals in each section, 1/4 by 1/4 by 2 1/4, 20 of these
for the major outside angle pieces on each selction 1/4 by 1/4 by 3 1/2 , 8 of these
now the minor angle pices in between the verticals, 1/8 by 1/4 by 3" like i mentioned above
now for the cross pices along the top of the bridge running horazontal, 1/4 by 1/4 by 4 1/2, 10 of these lined up with the vertical sections
it would be best if you draw this out on graph paper first, this one is not to scale but it is close, to assemble i used claps and glue, to cut the angle pieces i used a sander, trial and error till they fit in, you can build this and paint it in about 3 hours, it would be easier if you built 2 pices for the sides instead of 4 so it will line up better, so intead of 4 12" pieces you can use 2 24" ones, if you have any questions or want some more pics let me know, i could label a picture if you would like me to.