Loco in the middle is quite common in Austria - I've seen trains at Innsbruck formed of two push-pull trains coupled together the same way round, so that one loco is in the middle and the other at one end. The trains divide further east. It also happened briefly on the West Coast Main Line, when an experimental Motorail service ran. As the vans were not fitted for push-pull they had to be coupled at the rear, which meant the loco was in the middle on the southbound run.
Not quite a loco in the middle, but in poor weather the REP/TC formations on the Bournemouth line used to run with the REP "tractor" unit between the two TC units, rather than at the London end as was normal practice. This was, I gather, to ensure that, which ever way the train was going, the driven axles had at least sixteen undriven axles ahead of them to clear any ice or snow away.
The story of the forgotten Pacer on the back of a faster unit recalls the times on the Midland Main Line suburban lines when supposedly compatible units with hydraulic and mechanical transmissions were coupled together. There were several incidents of drivers in a hydraulic cab (which had an automatic transmission) forgetting there was a mechanical unit behind, and thus not using the gear changer provided, resulting in damage to mechanical units gearboxes as they were forced to run at speed whilst still in bottom gear. Eventually, the two types were given different coupling codes (red triangle, blue square) and allocated to different areas. Under TOPS they would become classes 127 and 115 respectively.
My favourite mixed formation? Back in the days when South Transpennine services were being run by a mixture of Class 123 and 124 units, I got a train formed of two Class 123 DMSKs, sandwiching a class 124 TBSK (the latter being one of the former non-driving motor coaches, and necessary to the formation to provide a brake compartment) As denoted by the "K"s, the unit was therefore entirely made up of side-corridor stock.
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