Poor maintenance combined with only doing the minimum required work just stores up problems for the future.
Past practice of designing and providing previous signalling schemes that are suitable only for diesel (or coal) trains (because it’s cheaper and at the time of the decision being made, there was no prospect of electrification of these areas) means that these need extensive work doing.
With these systems, it often makes sense to provide a brand new system rather than spending large amounts of money to alter the existing system, even if it is not life exprired.
You can only do limited amount of changes to signalling systems while the railway is open.
Just the signalling alterations alone are rather expensive.
The existing copper based telecom network also often needs looking at. Induced voltage from the OHL is not going to be appreciated by a telephone user…
Then there is the physical problems, like the gauge clearance. Stations, bridges, tunnels, viaducts, signals, aqueducts, other structures all may need changes. And some of these may be listed structures.
In addition, embankments may need to be stabilised before the OHL structures are erected. Drains checked or diverted. The list is quite extensive.
Unfortunately some work is often required after the OHL has been installed. On the GWML a number of signals needed to be renewed, as there was no longer enough clearance for staff safety if an electric train passed by (clearance to the pantograph).
This is vastly different to a line originally constructed with the future intention of providing OHL at some point. Or an existing line that has had work done, or systems provided with the future intention of providing OHL at some point.