Train Improvements

Add a designation on train tracks for dedicated waiting zones. I have currently built a four track wide siding for trains to loiter in until the destination is available, but I still get trains stacking two deep on a track, which blocks other tracks/stations. I know I can set exclusion zones and no stopping zones to sudo solve this issue, but id like to have trains seek out a dedicated rail yard to wait in instead of waiting on the main line.
Add a line specific exclusive zone to where only lines selected for that zone can enter it.
Add train trestle bridges to span wider and higher gaps. This would be great if it had a dynamic length where the material cost scaled with how large it was.
Add tunnels for both trucks and trains. Could add a brand new tunnel excavator to dig them out. Just think of all the cool subways we could build :)
Create station groups that persist for easier train line creation.