When there were still overnight sleeper services out of Kings Cross, some of the diversions they took at weekends were epic. The timetable was arranged in periods so that for about 2 months there would be one particular diversion in place, then for the next two months a second (different) one. The same train might get diverted off the main line several times in the course of its journey. Here's a few for you:
Sat 06/06/81
55017 22:15 Kings Cross - Edinburgh via Doncaster, Askern, York, Stockton, Newcastle, Carlisle, Edinburgh Suburban Line.
Sat 26/09/81
55016 22:30 Kings Cross - Edinburgh via Hitchin, Cambridge, Peterborough, Doncaster, Askern, York, Darlington, Leamside, Newcastle, Carlisle, Carstairs avoider, Edinburgh.
A total of 460.25 miles, my highest single trip with a Deltic.
Sat 14/11/81
40025 21:00 Kings Cross - Edinburgh via Peterborough, Sleaford, Joint Lines, Doncaster, Hare Park, Normanton, York, Leamside, High Level Bridge, Blyth & Tyne, Edinburgh Suburban Line.
When there was resignalling work taking place at St Pancras back in 1980, there were weekend diversions into Euston via Bedford - Bletchley.
Sat 31/05/80
45138 2120 Euston - Leicester via Bletchley, Bedford.
The 23:59 Sheffield - St Pancras was similarly diverted, re-engineing to a pair of 25s at Bedford (as the traincrew working it didn't know 45s), giving me 25095 + 25210 back into Euston.
Conversely, in 1991 when there was some engineering blockade on the southern end of the WCML, there were some weekend diversions of Liverpool - Euston services into St Pancras. These ran electrically hauled (or pushed) to Nuneaton, where an ETH 47 attached to take them via Wigston South Curve to Bedford, where the diesel was detached and the electric put the pan back up and worked into St Pancras. I did a couple of weekend trips, and was fortunate enough to manage an 86/2, an 87, and a 90 to St Pancras.
When the overnight Manchester - Gatwick Airport used to run on a Saturday night, the route of that train was never the same two weeks running. As well as the wide range of diversions that the LMR had to offer, it had a similar range of routes from Willesden to Gatwick on the Southern. A friend of mine once did it out of Manchester Piccadilly with a class 40 on the front, and it went via Latchford on the Low Level lines to Warrington, a route I never had.
Sun 22/11/81
There was some sort of engineering work affecting the route between Oxford and Leamington Spa, so the Paddington - Birmingham services were being diverted all day.
50001 11:36 Birmingham New Street - Paddington via Hartlebury, Worcester, Oxford.
50035 1900 Paddington - Coventry (- Manchester Piccadilly) via Oxford, Bletchley Flyover, Rugby.
This remains the only occasion on which I did the Oxford - Bletchley line throughout.