I would think that it is quite likely that we will see a Peak back on the main line before the end of the decade: It's not long since 45112 was taken off the mainline, and before that of course there was "Ixion", which ended the mainline preserved diesel ban in the nineties.
I just saw a Peak in a siding attached to an engineering train at Derby a few moments ago as we crawled past. (well it was an all over blue locomotive with four axles per bogie.... I don't know many other surviving classes that fit that description)
I also saw a random stretch of OLE.... maybe I am seeing things.