>>2060054Combos of AC & DC and different AC frequencies do not mix and need special equipment to isolate them from each other. Thameslink has 750V DC/ 25kV AC voltage switchover points at Farringdon and City Thameslink, which happens multiple times daily with no issues.
However, dual voltage rolling stock was more complicated to produce and less reliable in the past than now. Not sure BR would want to spend lots of money on niche rolling stock to avoid changing equipment specifications. Especially as line-side renewal was needed and conversion would be at its easiest during the renewal process. After the line was cut back to end at Hadfield and Glossop, the old gantries were used with the new 25kV AC equipment so it could be done. The cost of doing it the whole way would have been well beyond BR's budget.
Higher/lower voltages (e.g. 1.5kV & 3kV DC) can mix but only if the supply is lower than the train's rated voltage. It's probably better not to do this for very long or at a high power draw to avoid damage.
