I'm with @KLouis here. The fault is with the OP not Stansted. The trains are not "one-way". They take departing passengers to the piers and then return to International arrivals with arriving passengers. That is why there is no return for people who go to the wrong gate.
There are signs saying wait in the main terminal until your gate is announced. If the gate is not announced then no one will be at the pier. Unless, that is, you know who to ask. There is a manned security post at the entrance to the domestic arrivals pier. They have a display showing the gates so I often wander up there and ask.
There are often many reasons why the gate isn't displayed such as late inbound aircraft, another aircraft still on the allocated gate, etc, etc. However, I do get pissed off when they know what gate is going to be used but they don't tell you so as to keep you in the retail areas as long as possible.