Play scenario "Petrochemical Chaos" 27 times. 375000AP is needed and this one is the richest activity with 14000AP There are some (I think two) timetable shunting at Oakville automobile plant that gives about 10000AP That is how AP works, so without commuter trains it is quite hard to gain level up.
It is the exact answer to topic, but little crazy. By the way, if you play all timetables, scenarions, and tutorials once you don't reach level 20. Some of them have to be played again to reach route level 20.