At the moment when one levels up, you get a lovely green message saying 'congratz you've leveled up!' and that's literally it, no advantage no bonus no point to aim for it.
Instead, I fear might have already been suggested I'd expect, how about assignable skill points upon leveling up, like in most RPG games that have a leveling system.
Suggested format :
Upon leveling you recieve 2 skill points to be assigned to different areas, they have a small % effect on various things and cost progressively more, so for example 1 skill point for level 1, 2 for level 2, 4 for level 3, 6 for level 4 and so on.
The various areas;
Skilled Technician : Helps trains' servicability.
World Renowned : Gives a % bonus to gained prestige.
Areodynamic Genius : Gives % bonus to train speed.
Effecient Engineer : Grants % reduction to train, building and servicing costs.
Gearhead : Gives % bonus to train acceleration.
Charismatic : Passengers & goods give % bonus $.
Architect : Reduces construction times.
Lucky : Increased chance of gaining lottery ticket when collecting assoc member bonuses.
The actual bonuses can be small, somewhere between 0.1% and 0.5% so that don't have a large bearing on the game at all however will give people another way of personalising their style of playing, plus give a reason for leveling up.