The contract for the carriage of mail by rail is once again with DB Schenker now. DB provide the drivers for the trains, who are part of the link that drives the Caledonian Sleepers between Euston and Glasgow/Edinburgh.
The Class 325 Electric Postal Units (15 units operational plus 325010 acting as a "christmas tree") carry mail in York cages, which were once a common sight at many major railway stations around Britain. The driving vehicles can accomodate 42 York cages and the intermediate vehicles are designed to carry 44. By way of comparison, Royal Mails' newest "double stack" lorry trailers can accomodate 92 Yorks and the older double stacks can take 74.
In transit, the Yorks are held in place by fairly heavy duty straps and buckles which are affixed to the inner walls of the units. As has already been said, there is no sorting carried out en route, and no Royal Mail staff travel on board the trains. The York cages are manually pushed and pulled on and off the trains by Royal Mail staff at the mail terminals, with typically a small team of 6-8 workers entrusted to this task.
The mail trains run daily. The basic times at Warrington RMT, Monday to Friday, are as follows:
09:45ish 1S00 06:40 Willesden to Sheildmuir 8 car
16:45-17:00 ?? : ?? Sheildmuir to Willesden 8 car
19:09-19:30 15:31 Sheildmuir to Willesden 12 car Runs as 1M44 as far as Warrington, then as 1A97 to Willesden
19:46-20:02 16:40 Willesden to Sheildmuir 12 car Runs as 1S96
Arrive 20:20 17:50 Sheildmuir to Warrington 4 car Runs as 1M03
The same unit then departs as 23:30 Warrington to Sheildmuir running as 1S55
I think that they get priority in the pecking order of train flighting above most other things.