Joining two trains at Southampton - newbie's view

Call-on routes are allowed into the platforms at Southampton and at a couple of other locations. This is selected by the interlocking at routesetting time when the platform (or last track(s) in the route) track is occupied. Therefore, if a train is on its way to the platform, but hasn't yet entered it, do not overset a call-on route until the train is actually in the platform.

Note that the entrance signal to the route will only clear to a calling-on aspect when the following conditions are true:

That the platform track is occupied;
That the train to be called-on has been timed to, or nearly to, a stand at the entrance signal;
That the measuring track circuit(s) (where used) is/are occupied and the track circuit(s) in rear are clear. This gives you some protection against sending a too-long train into an occupied platform but is not foolproof. It is up to you to make sure that you don't put trains into platforms that would foul the station throat. Look at the Trains List to see how long each train is.

When the train reaches the location it then waits until the other train arrives and then joins. This will take a couple of minutes to achieve.

Huddersfield Control
Interlocking between a Signal (usually a platform starting Signal) and a Call-On Class Route leading up to the Signal.

122. I further recommend that a train standing in a through platform is not signalled to move before any second train has come to stand behind it. This would avoid (sic) the driver of a second train approaching a signal with a proceed aspect which does not apply to him (emphasis mine). This would also overcome the dangers from a second train following too closely behind a train which is departing, which was not a factor in this accident, but which has been in others.