Just seen a RAIL article which outlines GBRf and Colas’ plans for RHTT locos
Looking at the 37s in the Colas pools, some of the anticipated hires have already been transferred into the Colas class 37 pool (it's possible some more might be added later; I haven't tried working out how many have been added vs how many are likely to be required).
37s in Colas pools - current TOPS locations
LOCO POOL LAST LOCN DESTN
37057 COTS 573Q30C802 DERBY RTC
37099 COTS STNGDNSDG (Sittingbourne Down siding)
37116 COTS WORKSP DY
37175 COTS 570E602D01 BARROWHCR
37219 COTS 570E602D01 BARROWHCR
37254 COTS 573Q211702 DERBY RTC
37405 COFS 573Q30C802 DERBY RTC
37421 COTS DERBY RTC
37607 COTS BARRWHSD
37610 COTS WORKSP DY
37612 COTS WORKSP DY
37116 is shown out of service for overhaul as mentioned in post #97, and 37612 looks like it would require similar heavy maintenance before it could go back into service. The rest are shown in "normal" status.
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"Shakedown" was mentioned in post #95. I think the usual practice is for these "shakedown runs" to commence during September, once the RHTT sets have been distributed to their bases from York. The shakedown runs allow for proper in-service testing of the jetting and sandite equipment, which will have been out of regular use for a few months. During this time the sets will have received maintenance at York, meaning various things have been disconnected and reconnected - always a potential source of "gremlins." The period of shakedown running (usually running as required using the same schedules as the regular planned RHTT workings) is useful to iron out any problems and ensure the RHTT equipment is in good working order before the "sandite season" officially commences.
From what I remember, there's a phased introduction across the country (since the north gets colder before the south) where the Scottish circuits usually start running before the end of September, and the more southerly circuits start up maybe a week or so into October. Exact start and finish dates for different circuits can alter depending on the predicated weather and how quickly or slowly the leaves are expected to fall compared to a "normal" year.