I made this Tower Bridge for my son and his wood train.
The idea is to mount (hotclue or screw) one wood train rail between two parts of these.

The parts are optimized for 3D printing. So you could print the bottom part without any support simple rotate the bottom part.

The top part need some support for the connectors. You should rotate the top part as well.

Infill depends on your needs, if you want that you kids can play with them without breaking them then you should use some infill.
If no kids will play with them, then you don't need any infill at all.