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.

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.

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.

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