The ability to drive (as a train passenger AND the train driver) to other places eg. Rochester to London Victoria with joy.
I think what he means is that if you boarded a Victoria bound train at Rochester and stayed on it, you could end up at Victoria if you have the right DLC
For me, it's Eastcoastway & BML if the Wivelsfield connector is built. Riesa Dresden with Dresden Bodenbach Prag if made and I'm using the Austria Hungary German names for Decin & Praha (Prague). That way some of the Roderau Dresden freight locomotives end up being a DB BR180 CZ371/372
The issues with this are what happens between DLC, so in the case of Rochester to Victoria the SEHS route ends between Rochester and Sole Street and only starts again within a mile of Victoria. How would the game render that? And what would happen to those who don't have relevant DLC, or where there's a difference in time period? Too many variables and not enough payback for me