The thing is, even with tilt, what are these trains going to do North of Darlington, mess up the paths?
I mean, given IEP is non-tilt, I assume it will be limited to 75 mph over the Durham viaduct and the various sub-125mph speed limits North of there, along with whatever stock runs there. Surely introducing a 125 - 140 mph tilt train would screw up all the paths in the area (although I guess the frequent Morpeth Pacers do this already)? Maybe North of Morpeth there aren't enough trains to cause a problem (up to 2tph EC, 1tph XC?).
I also assume that 75mph over the Durham viaduct is a structural issue with the viaduct, as opposed to a bend issue and everything will be limited to 75mph over that, tilt or no tilt.
I assume the use of Pendo stock is to make a large stock order with some WCML Open Access, with balises already installed. Running an OAO is expensive as is (didn't Grand Central lose money for its first 7 years of operation?), paying for TASS balises for 1tph seems like a rather disproportionate outlay.