I did the Inverness one southbound in the seated carriage many years back, had an open return for work so thought I’d give it a go. Just myself and a family in a declassified First Mk2 carriage, but they had a noisy baby which meant sleep didn’t happen. I jumped off at somewhere like Lancaster or Preston at four or five in the morning with a three hour wait for a local train down to Manchester so I could then get home on a transpennine service. Sub-zero temperatures and conveniently none of the waiting rooms were open yet, although eventually a staff member took pity and opened one up. Bit rough really.