Are you counting the London Underground? If so, Euston to Waterloo's a route where the plausible accessible options I can think of are a bus, with the least bad Tube option I can think of being double-backing to Northwick Park(!)* (itself needing getting from Euston to Euston Square) and then changing at Finchley Road. The other direction doesn't look much better (I think changing at Bank, and Moorgate or King's Cross St. Pancras for Euston Square due to Euston Tube station not having any step-free entrance, and Euston Square westbound only).
*The accessible Tube map has it rather than Preston Road highlighted despite both being single island platforms, which might be down to gaps between platforms and trains.
I think that's a case where the (shortest) minimum connection time of 50 minutes wouldn't be sufficient. Using the TfL journey planner suggestion and removing buses as an option (due to Maltese cross tickets not being valid), it suggests at least 49 minutes northbound (changing at Green Park, King's Cross St. Pancras and Euston Square) and 65 minutes southbound (I misread the map at first and it's 'only' necessary to go as far as Wembley Park for an accessible change).
Clapham Junction and Willesden Junction seem more accessible, so avoiding the Tube altogether could be a desirable option.
