You haven't explained why somebody else should get a worse service? Why not just let everybody queue in order of arrival?
Although not justifying the whole thing there could be times when you'd want to be able to give certain people an advantage when getting then into a train.
For instance those who need assistance (that's not too say that you couldn't help them under another method).
Personally I like priority boarding when it comes to aircraft as I want to spend as little time on the plane as possible and so I'm quite happy sat waiting for everyone else and then board near the end. The problem with that in the trains is that not everyone has a booked seat.
If you could ensure that lots of booked seats are occupied before you let those without a reservation on then it would make it easier to find a free seat. As such I would suggest that you could only be in the priority boarding line with a seat reservation.
I'm not sure that such a ramp queuing system would work though, as if you have a train with circa 500 people come in to a platform then loosing a significant width of the ramp for passengers waiting may not be an overly good idea. Especially given that you'd be looking at about 2m in width for two lanes.
You may be better off holding your priority boarding passengers elsewhere (maybe with a free drink) and then bringing them down the exit side.
You could then do the same from the first class lounge for your first class passengers, probably trying to time it so that both groups are on the ramp at a similar time.
You could then open up the main gates to everyone else, but have a priority/first check in at the top of the ramps to allow those along later to use the up ramp to join the service whilst the bulk of the passengers join the service.
As it gets closer to the departure time, then close the priority/first check in and just let everyone down (as there's likely to be less congestion anyway).