Now that I have more time, I would like to expand on my previous answer to the gauge question.
Firstly to reconfirm, C1 relates to the standard vehicle gauge for rolling stock. Only C1 gauge is recognised for the purposes of gauging as ALL rolling stock must fit within this gauge.
C1 vehicle gauge is based on passenger vehicles which fit within the following criteria :
Length – 19 354m over the headstocks
Distance between bogie centres – 14 173m
Bogie wheelbase – 2591mm to 2967mm
Both vehicles with conventional sprung bogies, as well as those with air suspension are included within the C1 gauge, albeit there are some slight detailed dimensional variations but none which are of relevance to this explanation.
C2 is not a gauge I recall being used.
C3 vehicle gauge simply denotes that the vehicle is a 23m long Mk 3 vehicle. It was introduced in BR days as some standard routes had not been specifically cleared for the use of MK3 stock. For example Bletchley to Bedford was not cleared for Mk3 stock until 1985, although Mk 3 stock had operated along the WCML for many years previously. There is a belief that C3 is a gauge. It is not. It has no relevance in gauging whatsoever as a C3 vehicle is designed to remain within C1 vehicle gauge.
C4 vehicle gauge was originally used to denote a passenger vehicle which was designed to tilt within the C1 vehicle gauge, and was originally applied to the Mk4s although as various people have stated, these vehicles were never introduced as tilting vehicles. The designation is now referenced against the Class 390 Pendolino stock.
There are two other gauges which must also be considered.
The first is the Eurostar Class 373 vehicle gauge (Eurostar Gauge) sits by itself and is only applicable to those trains.
The second is UK1 gauge, which is the new gauge to which is referred to in the Technical Specification for Interoperability (TSI). Any train that it is proposed to operate into the UK through the Channel Tunnel must meet the vehicle gauge requirements in order to be operated within the UK.
All new infrastructure is built to allow for UK1 gauge rolling stock, and as existing infrastructure is renewed, where possible the clearances are designed in. The structure gauge which applies to these structures is in no way directly related to UK1 but does allow UK1 vehicles to operate, and it is very important that people do not confuse vehicle, loading and structure “gauges”. They are three very different items.
Structure gauging is a complex subject even for the experts and I hope I have managed to simply a complex process without causing too much confusion.
Any question, please ask and I will do my best to reply, or find out the correct answer.