80Xs supposedly can only come in an odd number of vehicles (5,7,9,11??), I'm not sure why.
5 cars is good for off-peak, and 10 cars is loads for peak services. EMR can't justify 9/10 cars all day, and a mixed fleet would be more difficult operationally and would require a much larger depot to deal with longer units.
Looks good, vast improvement over the IET interior.
If the moderators would allow me just a tiny bit of leeway to clarify the question of formation lengths, the DfT's Intercity Express Programme specification for what became the 80x series of trains set a maximum permitted length for a formation at 312 metres - ie a 12-car formation of 26m coaches (see page 19, section 3.3) and a minimum length of 130m - ie a 5-car formation.
This note suggests that any length of train within those limits is permitted - though obviously local infrastructure like platform lengths at the likes of St Pancras, King's Cross and Paddington can also play a part in decision-making.
It must be possible to add Intermediate IEP Vehicles to an IEP Unit subject to the IEP Unit still being no greater than the maximum length identified in TS223.
In Annex D of the specification (sections 2 & 3, pages 77-79) there is a set of train formations, showing what facilities would be required in 5-car (including a high-density version without a kitchen), 8-car, 9-car, 10-car, 11-car and 12-car formations. The last three were described as 'indicative for pricing purposes only'.
The full specification is at https://assets.publishing.service.gov.uk/media/5a79b82e40f0b63d72fc7f58/tts-redacted.pdf
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