Their general reliability and good braking characteristics made them popular with drivers.
Vehicles 50699 / 50701 were noted in the stock shed of Swindon Works, on the 3rd Nov. '57. Noted in the Carriage Works on the 29th January '58 were 50655-60/2/3, 59259-72, 50698/700/6/7/15/7. Some of the vehicles were coupled together on trial: 50698/50700/50707/50706, and 50658/59259/50657.
The first sets delivered went into traffic on the WR on the 10th March 1958. They replaced steam between Cardiff and Bristol and between Birmingham and South Wales stations on the 08:05 Cardiff to Bristol, 10:22 Bristol to Cardiff, 16:47 Cardiff to Birmingham, and 17:40 Birmingham Snow Hill to Cardiff. They also replaced the Class 126 Inter-City sets on the 12:10 and 20:15 Cardiff to Birmingham and 12:25 and 20:30 Birmingham to Cardiff. This allowed the 126s to go north to join the rest of the fleet based at Leith and Ayr. Sets noted on these new Cross-Country workings were from the range 50653-9 / 59257-62 / 50698 - 712. The Cardiff - Bristol working was operated by two 3-car sets, and required two sets of catering staff to man both buffets.
The Class 120s provided passenger facilities normally associated with the Inter-City trains, with more economic operation where the traffic was not large enough to justify the large formation.
On the 21st April they started between Oxford and Birmingham. The fastest left Oxford at 11:20 calling only at Banbury, Leamington and Warwick, arriving in Birmingham at 12:51. The 23 miles from Oxford to Banbury was run in 25 mins start to stop, and the 20 miles from Banbury to Leamington in 22 mins, both 'fastest-ever' timings. Other times were 08:40 Wolverhampton - Oxford (which ran on Mons - Fris, and also on Sats till the 10th May), and a 09:10 Birmingham to Oxford (this was possibly extended to being a 08:35 from Wolverhampton) on Sats from the 17th May. These were 3-car workings, but changed to 6-cars from the 17th May. There was also a through diesel train from Didcot to Wolverhampton (Mon - Sats), leaving at 07:04 and calling at most stations. This was a 6-car working, and the return left Wolverhampton at 16:40.
Noted in Swindon Works on the 4th May '58 were 50665-7/96/7, 50715/8/9, 59273-8.
An unadvertised excursion on Sunday 1st June '58 ran to Portsmouth Harbour, the first appearance of a diesel mechanical set there. The 3-car 120 ran from Hanwell via Basingstoke & Eastleigh.
As deliveries continued, it allowed further services to be taken over at the start of the summer 58 timetable on the 9th June. They were introduced between Birmingham and Cardiff via Worcester and Hereford, and between Shrewsbury and Hereford. On both lines the number of trains were increased as was their average speed.
Between Birmingham and Cardiff the intermediate stops were generally Stourbridge Junction, Kidderminster, Droitwich, Worcester Foregate Street, Malvern Link, Great Malvern, Colwall, Ledbury, Hereford, Abergavenny, Pontypool Road and Newport. The average time was 3 1/4 hrs. The existing 08:00 from Birmingham was accelerated 15 min; the 09:45, which now started at 10:00, 16 min; the 12:00 was a new train in 3 hrs 10 min; the 13:00 also in 3 hrs 10 min, speeded up by 44 mins; the 17:40, which previously terminated at Malvern Wells, replaced the 17:00 with an acceleration of 40 mins and provided with a buffet car. The 20:00 was another new service, as was the 07:50 from Hereford to Cardiff, the latter with buffet car facilities. There was also similar improvements in the reverse direction, leaving Cardiff at 07:40, 08:35, 10:40, 12:42 and 18:40. There were also additional early morning and late evening trains from Hereford to Cardiff and Birmingham.
The summer '58 timetable also saw the Birmingham - Cardiff 'Inter-City' service via Stratford and Gloucester slightly de-accelerated, due to additional stops being made, mainly at Statford and Cheltenham, and there were now no non-stop runs between Gloucester and Birmingham.
Between Shrewsbury and Hereford there was a complete transformation of the existing timetable. The passenger service was withdrawn from all intermediate stations and halts except Church Stretton, Craven Arms, Ludlow, Woofferton Junction, and Leominster, and the Class 120s covered the 51 miles in 75 mins inclusive off all these stops, except that at Woofferton, which was made by two services in each direction only, other stops being made by Ludlow - Tenbury branch trains.
Compared with the previous stopping trains from Shrewsbury at 06:25, 10:10, 13:15, 15:55, 18:55 and 20:22, averaging 110 mins on the run, there was trains at 07:00, 08:50, 10:40, 13:15, 15:05, 16:05, 16:50 and 18:40, also at 20:30 and 22:30 to Ludlow, besides the non-stop north to west expresses. The 07:00, 08:50, 13:15 and 16:50 had buffet cars, and the 07:00 and 16:50 were run through to Cardiff. Some of these trains made good connections at Hereford with the new Birmingham - Cardiff service, and also served as feeders to trains from the north to the west of England. Northbound, the new trains left Hereford at 07:05, 08:10, 10:45, 12:15, 15:00, 16:45, and 18:45. All but the 08:10, 12:15 and 16:45 had buffet cars, and the 08:10 through from Cardiff at 06:40.
On the 11th June '58 vehicles 50717 / 59273 / 50666 and 50665 / 59275 / 50719 operated a Bristol Division permanent way outing to Totnes. The party then went by river to Dartmouth and were met at Kingswear by the train, which returned from Torquay at 18:30. It was also claimed that the trip was run for 'experimental purposes'.
The Birmingham - Hereford - Cardiff sets were noted as having an ordinary coach attached at the rear when required. An example was on the 18th June '58, when the 17:40 from Snow Hill was formed of 50697 / 59276 / 50671 for Cardiff and 50657 / 59263 / 50712 with SK W5865W for Hereford.
Noted at Swindon on the 27th July '58 were 50673/4 and 59280/1.
Around July '58 120s were doing the odd 'filling-in' turn on Wellington - Lapworth stopping trains. The 84E shed plates which appeared on the leading buffer beam of several sets at this time began to be removed and trimmed top and bottom to fit on the underframes of the leading cars.
On the 30th August '58 a day excursion from Birmingham to Paddington was worked by 50657 / 59263 / 50712 and 50719 / 59275 / 50665. The return from Paddington was at 20:25.
The winter '58 WR timetable saw the new diesel services introduced in the summer timetable practically unaltered, except all buffet facilities were withdrawn.
Vehicles noted at Swindon on the 16th Nov. '58 included 50683-7, 50734, 59291-5 along with three Class 119 sets.
A crisis arose in the London Operating District of the WR towards the end of '59 when it was attempted to work the diesel timings with steam trains resulting in deplorable timekeeping. As a temporary measure sets were borrowed from Tyseley and Cardiff. In consequence many trains, advertised as diesels, between Swansea, Cardiff, Bristol and Birmingham were steam hauled. All but two Cardiff sets were returned within a fortnight when twenty Cathays suburban sets were sent to the London District.
Sets on loan included:
TYS 500 50650 / 59256 / 50700
CDF 507 50658 / 59261 / 50711
CDF 516 50609 / 59277 / 50718
CDF 528 50683 / 59294 / 50735
BL 516 50692 / 59300 / 50743
BL 517 50693 / 59299 / 50738
The class also worked services from Crewe to Lincoln, Birmingham to East Anglia, Manchester to Blackpool.
In the late '60s, 22 sets were transferred from the WR to Derby, to replace the less successful Cravens and BRCW units. In 1977 seven sets were transferred from the Cardiff to Chester in exchange for Gloucester build Cross-Country sets, followed by a further four by 1980. All the Chester sets (ex-Central Wales 2-cars) and some at Derby had their trailers replaced by Met-Camm trailers, although other types were also used, such as BRCW 59206 and Class 107 59794.
In 1980 the Inverness sets moved, 4 to Cardiff (CF) and 3 to Ayr (AY). The following year the Ayr sets were transferred to Derby, and NH received 2 sets from the WR. By 1983, only 2 sets remained at CF, with 31 at Derby, 17 at NH and 14 at Chester. At the beginning of 1986, Derby had 25, NH 16, Chester 4, Cardiff 2 and Ayr 7.