IIRC they used to operate HSTs into Queen Street, but had to swap as the Wiring on the Airedale line deteriorated, so it can no longer support a 91 and the London-Skipton service is now an HST.
Correct that the ECML Glasgow services used to be HSTs, and Skipton services are HSTs running under the wires all the way from London, but to clarify and add to this - there was no connection between the two events:
* Glasgow services prior to the ECML electricfication in 1991 were infrequent 2+7 HSTs, which only just fitted in at Glasgow Queen Street, and even then only in platforms 6 and 7 IIRC. The pointwork and tunnel mouth at Queen Street prevent any longer trains using the station and would be very expensive and difficult to fix.
* As the 91+Mk4+DVT (225) combination would be too long to fit at Queen Street, the Intercity sector hit upon the idea of electrfying Haymarket to Carstairs quite cheaply. This linked the ECML and WCML electric networks and meant that through services from the ECML could continue to run to Glasgow, but into Glasgow Central instead (and mostly through open countryside rather than the small and medium sized towns on the route to Queen Street).
* Therefore, from the start of full ECML electric services, the Glasgow services no longer served Queen Street and were formed of 225 sets - very rarely did HSTs run through to Glasgow, generally only because of stock displacement or engineering work. The service to Glasgow increased from a couple of trains a day to one every two hours or so.
* The electricfication to Skipton and Ilkley was unrelated to the ECML project - it was a WYPTE project to improve the quality and capacity of the local services on the Airedale and Wharfedale routes.
* Prior to ECML electricifcation there were a few through services to Bradford and to Harrogate, but none to Skipton. The WYPTE electrification was seen as a way of improving InterCity services to Bradford, but the idea of running through trains from London to Skipton was formed almost as an afterthought to the WYPTE electrication, not as an integral part of it.
* As a result, the electrical supply was designed only to allow basic EMU traction out to Skipton. When a 225 set was tested out to Skipton, it caused an unacceptable drop in the current available in the overhead, such that other electric trains beyond Keighley would pretty much be stopped.
* Therefore to keep the plan of through trains to Skipton they would either have to be loco-hauled from Leeds, be HSTs, or if 225s were to run the timetable would have needed complete recasting to only have one train running between Keighley and Skipton - leading to the situation today with HSTs forming the Skipton service.
