Just to pick up a few points.
While Cl321's used to be the EMU of choice to deputise on London-Norwich runs, this has fallen by the wayside. I'm not entirely sure why Cl317's took over this role but, with last year's summer timetable and the end of loaning units out to c2c and Thameslink, there seemed to be units to spare. These were stabled out at Colchester and operated by Mainline traincrew from the same location. Since the end of that timetable, the requirement for Cl317's on the WA division has increased. Therefore, I would be very surprised if there was still a specific allocation of standby units (although we do keep at one on standby at Liv St should the need for it arise).
There are technical issues over the choice of substitute. Currently, Cl360's are not cleared to operate north of East Suffolk Junction, which puts their northernmost limit at Ipswich. Like the Cl321's, Cl317's are fitted with CSR only and do not carry NRN equipment. However, I believe that the real deciding factor is down to the traction knowledge of the Mainline crews that are expected to operate them. They "sign" Cl317's because of the pool of substitute units used last summer, but I don't think that they "sign" Cl321's.
As for the relative acceleration of a Cl90 compared to other traction combinations, this is a complex topic and not just down to maximum speed or power-to-weight ratios. Mk2's and Mk3's were never designed with push-pull operation in mind. Therefore, there is an amount of "give" in the couplings. This isn't to much of a problem when pulling, but it requires careful driving when pushing to avoid any stressful or uncomfortable "snatches", meaning that they have to accelerate gradually. Either way, an EMU is certainly capable of maintaining Cl90 timings.
The IC225 sets are the only trains designed from the outset for push-pull operation and, from my experience riding them, it seems that the operation of the Cl91 and the train's brakes have been carefully tuned so that they work together. It always seemed to me that the train's brakes were bled out slowly rather than the air being dumped in one huge gasp, while the Cl91 gently applied pressure. It was always possible to tell when the Cl91 had been swapped out for a Cl90, as there was always a hard thud from the back as the loco seemed to be trying to accelerate hard against the train's brakes (although they may have improved the situation since the mid 1990's when I last experienced this form of traction). Once the brakes were all bled out, the Cl90 would then accelerate the train in short order up to, presumably, 110mph, and almost certainly attaining that speed more quickly than would have been the case with a Cl91 providing the motive power.
HTH
one TN
