Usually, bikes aren’t allowed on San Francisco-bound BART trains during peak morning commute hours, or back to the East Bay in the evening. And they’re not allowed in the 19th Street or 12th Street stations during commute hours at all. But this week, BART has opened up all hours and stations to bikers. It’s a trial period, and to make it work, BART officials and cycling groups are urging to riders follow the rules: no bikes on the first three cars during peak hours, and no bikes on crowded trains.