at overall, engine and tender, the s1 was the longest reciprocating steam locomotive ever; it also had the heaviest tender (451,840lb / 205 tonnes), highest tractive effort () of a passenger steam engine when built and the largest drivers (84 inch diameter) ever used on a locomotive with more than three coupled axles. the problem of wheel slippage, along with a wheelbase that was too long for many of the rail line's curves, limited the s1's usefulness. no further s1 models were built as focus shifted to the much smaller but more practical class t1 in june 1940. design of the 4-4-4-4 t1 and the 6-4-4-6 s1 occurred concurrently, however, the s1 was the first produced.