Cool railgun shooter. The Mech looks great and so are the turrets and tanks. Glad there are repair spots. Enemies do a ton of damage to the player, maybe a little too much. Enemy health bars don't fully go down, might want to see if the numbers go to zero or not.
Not sure why your file size is so large, you don't have crazy textures. Did you build one huge map or did you move terrain/object after they are offscreen? I have a couple of high texture materials that I had to lower the file size for to make the WebGL version small enough. I did lower them a bit much and would like to bring them back up a little but was running low on time.
Might want to create a different incentive for the player to go away from the train, like to rescue some survivors in the distance. Or to chance picking up extra resources. To give variation to the stop/fight locations.