To start, put in enough money to cover your guaranteed transit expenses. If $150 per month is your total train expense, then that's your answer. If you later realize that you take more weekend leisure trips than anticipated or realize that you don't need to be in office everyday, you can always increase or decrease your contribution level anytime you want through wageworks. (In that way, it's different from your healthcare selection, which you can choose/change only once a year.)

Also see if your employer makes any contributions to your wageworkers account to supplement your contributions.

Aside from that, I'm not comfortable with providing advice since I don't know enough about your circumstances to tell you how to manage your money.