As noted before,
HOBC▸ is very long - or at least can be (depending on what "maximum" means) - this is from the Western Route Sectional Appendix:
High Output Ballast Cleaner (HOBC) and Track Relaying
System (TRS) Trains
These trains are authorised to transit between their operating bases and engineering possessions in excess of
the normal route length limits on Western Route provided that a suitable train path has been identified.
The train identification used and maximum lengths (including locomotives) are as follows:
HOBC 6Y11 127 SLUs 811 metres 887 yards 2659 feet
TRS 6Y33 117 SLUs 744 metres 813 yards 2439 feet
The HOBC and TRS may also exceed the maximum permitted single engine load between the locations listed
below on Western Route. In these circumstances the train concerned must operate with a locomotive at each
end. The rear loco is authorised to apply power as directed by the lead driver to assist as required in the
negotiation of inclines between the mileages shown. In these cases both locomotives must be manned as per
Train Company manning agreements and equipped with back to back radios.
The list of track sections where two locomotives are mandatory include a lot more than the usual very steep gradients, and several stretches of main line, so I think it will always have two in practice.
Realtime trains shows no freight working with HOBC's customary ID of 6Y11 through Thatcham on the 13th, and only routine pathed runs with other IDs. The only "heavy" that evening passed at about 19:15.
