Are there any duties currently undertaken by 37s or 20s (or indeed any other aging loco) on lines that are RA6 or below that could not be handled by a Class 66 running slowly over the problem bridges? Given passenger duties are almost exclusively MU, and light pick-up freight doesn't really exist anymore, I really don't see the need for a specialist fleet to do a like-for-like replacement. Ad-hoc moves can be accommodated with a bit of preparation and organisation, and any regular operations would probably justify work to fix the problems anyway.
If they are used, they are needed. And this is still replacing 2 with 1.
Very wrong. If you have it and it can do the job, you may as well use it. But you aren't going to spend £<large amount> per loco unless you really need it to do that job, and certainly not if another loco you already have could cover it quite happily.
Well they cleary don't have another loco they are comfortable with in the role....
Operating one loco does not mean they would not operate another. Class 66s are cleared in the Sectional Appendix on routes below RA7 to cover ad-hoc duties like possession trains.
But clearly not every route. If they could find something else, they'd use something else, surely. To me, this ends in two ways, inserting a new powerplant into the old locos, or building a new loco to replace them all.
You're missing the point. At the moment, they don't need anything else. But when they are faced with the choice of spending possibly £100m a loco or doing some paperwork and asking the driver not to go beyond 30mph, I have a hunch which option they'll take for the sake of 1 train every 4 weeks.
And you are missing the point that they'd already be doing that most likely. I think there is more to the problem than either of us can see.
I'll be honest, I have no idea how much stock costs. Once you factor in a specialist design fro UK loading gauge and RA5, then testing and bedding in it will be more expensive than getting clearance for existing locos