The railcar has two six-cylinder Meadows diesel engines, one at either end.
"One of the motors needed to be replaced, so we had to get an engine overhaul, gear box overhaul and other bits and pieces done," said Mr Jones.
The railcar was lifted by cranes and suspended above the rail track, by about 1.8 metres.
Then the bogie, the carriage base, with the "old sick motor", got pulled out from underneath the railcar and the railcar was lowered back onto the track.
The old engine was removed from the bogie and replaced with the new one and the railcar was lifted back up so the bogie with the new gear could be put back underneath.
Mr Jones said the job took about three hours.
"We don't do a lot of runs with it so it [the engine] will be good for about 20 to 30 years, I'd say.
"That other engine is okay, but it will probably need replacing in the next five to 10 years."
On occasion, the RM31 takes excursions, such as to the Art Deco Weekend in Napier where it takes people from the station to the port and back again.
The RM fleet of six finished service on the New Zealand railways in 1972.
Mr Jones said they travelled all over the North Island, but more so in the lower and central areas, including on the Wairarapa line.
All six were named after ancient Maori canoes, "Tokomaru" belonging to RM31.
The Silverstream Railway has RM30 "Aotea" and RM34 "Tainui" and the Waikato Branch NZRLS has RM32 "Pangatoru".