If you can arrange to enter tunnel portals and bridges on the straight, your life will be easier..
If you enter on a curve, the track will 'walk' over time, towards the outside of the curve.
To counter this, try to enter on the straight, and secure the approach with a couple of screws, about a foot apart.
You can let most the rest of the track 'float in the ballast.
Oh! Over a season, you will find all those expansion gaps, gang up and get together.. So some track ends up hard against the next section, and in places the gap gets huge! - Don't ask!
All part of the fun of looking after the 'permanent' way.
PhilP.