You could use a lock state cell. Detector rail>State Cell>State Cell>Boarding Track. That way the first State Cell runs only once the train arrives, and after 30 or so seconds sends a signal to engage the second State Cell, to launch the train.
The other option is a timer lock set up, which would require a slightly expanded train and one tank cart. Place a tank detector under the front train, facing a NOT Gate. When the track detects the tank cart, it disables the not gate, which unlocks the timer, and lets it run. When the train leaves, the NOT Gate locks the timer so it doesn't run.
Of course if you're REALLY buff for resources, the super train is an insane trick. Three or four locomotives, connected to a train the same length as the tracks. The only thing that really happens is that the loco's move, and push the train around the track. If you want to get on, you stand beside a boarding area, get sucked in and then as the train moves around you'll eventually reach the destination. I'm not sure if it works with electrics, but you'll want as flat a track as possible. That also makes it a lot harder to get hit by the train because it's harder to get onto the tracks.