But it is a question that needs to be answered before stock for the route becomes a pressing issue. To the casual observer often it seems that there isn't a huge amount of forethought given to clearing routes for different stock.
As a casual observer, I'd say that it isn't a problem for a few years though. At some point, someone will start to look at 156s and say that they aren't going to be able to continue but there isn't going to come a day where they are just taken out of service. Equally, someone at Barrow isn't going to find they have a 195 available and not a 156 and send it to Carlisle - they will just cancel the service because the diagram may involve staff at another traincrew depot on the coast who are not trained. When the time comes for replacement of 156s in ten years or so, they will consider what changes need to be made to accommodate 195s and how much training is needed.
I agree that it all this interesting and if some could elaborate on each of the restrictions and why they apply it would be great. Perhaps a locally based expert on the line's operation.
However, if we look to other parts of the network, an operator decides that they want to use a particular type of train on a given route, speaks to Network Rail and there is then a process to ensure they can operate - eg there was a process to get 165s and 166s cleared over the western network which involved ride-height modifications, changes to platform clearances etc. Most trains can be made to fit in some way or another but that work isn't sanctioned until shortly before it comes a pressing issue, not as a nice to have.
For the time being, 195s are fully utilised on the Northern Connect network, not primarily on regional, secondary or local routes and 156s have just gone through a overhaul.