I have not yet tried to merge routes in TRS22 but from experiences in merging in TRS19 and Trainz Plus it may not actually be a problem at all. It does depend on the size (number of baseboards) of the route or routes you are trying to merge.
When merging large routes (large number of baseboards) the "problem" is actually caused by the display area in the merge map and the zoom function. The merged route (and sometimes neither route) will not always be in the centre of the display area at the current zoom level so it may not be visible.
For example. This is what I see when I select 2 large routes to be merged.
The
!symbol on the terrain icon (top left) indicates that the merge has occurred with an overlap.
First step: Zoom out (mouse scroll wheel) as far as possible.
Second Step: The two routes are now visible. Use the right mouse button to click on the merge overlap zone (the red coloured baseboards) to centre it in the display.
Third Step: Use the zoom function to zoom in.