They were allocated new to the LMR, spending most of their time at Chester. The first deliveries went to traffic in the Llandudno area, allocated to Llandudno Jct. The last four sets were transferred to the WR, and worked in the Cardiff and Reading areas. Although considered non-standard, their use of standard BUT equipment and blue square coupling code allowed them to operate with other classes. They spent some time in the Watford area, working the St Albans and Belmont branches after the demise of the ACV/BUT railcars.
In June 1958 Class 103 sets, along with GRC&W Class 100 sets, took over steam services in the Walsall area.
50409 / 56164 went to Watford in December 1958 and was noted working the St Albans branch on the 13th.
The Class made their debut on the Harrow - Belmont line on the evening of 18th December 1958 with 50414 / 56169. Sets 50411-4/56166 transferred from Walsall and Monument Lane then monopolised the service.
From mid-November 1958 until at least Easter '59 three sets were required on Sundays to operate a shuttle between Wembley and Harrow during track work on the electric line between North Wembley and Wembley. This meant that the ACV vehicles were back in action on the St.Albans branch. On the 24/3/59 50413 failed before leaving the shed for the Belmont line, and a couple of ACV vehicles were quickly prepared to take it's place.
Easter Sunday '59 saw 50408 & trailer work with two Class 100 sets on a W720 Walsall - Euston excursion.
Trouble with cracked bogie frames on the 103s at the end of '59 caused an acute shortage of DMUs, resulting in the ACV vehicles re-appearing on the St Albans and Belmont branches, and the unreliability of these meant that the services were often cancelled. No services ran between Harrow and Belmont after the morning of 23/12 till a Park Royal set returned on the 29th. Further problems just over a week later saw steam locos brought in. On the 12th January 1960, the only set left was 50413 / 56169, which was out of action, and by mid-Jan it was still steam operated, with ACVs as standby.
Set 50412 / 56167 was the first noted back on the Belmont branch, on the 13th February 1960, joined by 50414/56168 and 50413/56169 over the next couple of days.
In the winter of 1965 a set operating the last Up train from Barmouth to Machynlleth was misrouted at Dovey Jct into a siding. The set went through the buffers and into the DTC sank into the bogland. It was recovered by crane the following day. More details and images can be found at www.rmweb.co.uk/forum/viewtopic.php?t=7628.
On the 8th May 1972 an oil train 'ran away' and crashed in Chester General station. M50407 and M56154 were in an adjacent platform and damaged beyond repair.
The bodywork of the vehicles became troublesome in service, seeing many withdrawn early. By the end of 1972 there were only 12 power and 14 trailers left in service.
Vehicles could on occasion be seen running as part of hybrid sets, particular in their later years. In their final months they still made almost daily appearances on the North Wales line from Chester to Rock Ferry, normally power cars with 101 trailers. On the 20th November 1982 50409/56152 worked the 10:20 Chester - Manchester Victoria and 12:45 return, coupled to Class 101 set 51178/56335. 56151 was found wokring with Class 101 power car 51191, and awaiting disposal at Chester were 50398, 50400/2 and 56158/64. Found at the rear of the station was 50405 (not officially withdrawn), 56161 and Class 120 centre cars 59278/91.
The last two vehicles were withdrawn in February 1983 - power car 50409 and trailer car 56152.