How to get the train onto the third island before you unlock it:

Get a train and accelerate the train up to maximum speed (shaking) - don't let this happen til you are exiting the tunnel from Whetstone.

Allow the train to keep shaking for that tunnel to the next keep going (it shouldn't derail).

Once you get to the barriers the train will slow down, but not enough to stop.

By the time it stops you will be past the barriers, then keep going.

Remember the same applies for on the way back, don't forget the police, but in a train they can't kill you.