You can't. In OpenBVE, currently it is only possible to follow the first car.
Oh boy, we are really outdated.
I won't lie - I thought it wasn't possible too. But I remember bumping into a video some time ago where someone rode the R142 (or R142A) in the conductor's cab. So I did a bit of researching today and it turns out that yes, it is possible. Ever since
version 1.2.11.15 was released, you can now
specify the car that you're in in the Cab section of the train.dat. That means yes, you can now be the conductor.
Check the
Developing for openBVE page to see what you need to edit.
For those that still don't understand what to do, I'll make it simple:
1) open the train.dat,
2) locate the Cab section (#CAB),
3) find the fourth number and replace it with either 4 (for 8-car sets) or 5 (for 10-car sets). If there's no fourth number, this will be the fourth number.
One more thing: I think the result for generic, negatively-built exteriors might have a different result compared to exteriors that are centered. I haven't tested it out yet, so try it out and see what happens.
