There are several well attested examples of very long trains; especially excursion trains. E.g. An excursion from Ripon to Scarborough in 1850 collected passengers along the route and arrive in Scarborough carrying 1,600 passengers crowded into 49 carriages and pulled by two engines.
Reference:
Major, Susan - Early Victorian Railway Excursions: The Million Go Forth (Barnsley, Pen & Sword Transport, 2015) p5