My housemate was a student in Sunderland around 1989-90 but we lived in Newcastle. He used to catch the train to lectures occasionally and the daytime service during the week was generally 4 trains per hour I recall with a mixture of expresses that only called at Heworth, for the Metro interchange, and stoppers, which called at all stations between Heworth and Sunderland. There were not as many stations as now and Brockley Whins was called Boldon Colliery, as there was or had recently been one near the station. I think one of the fasts ran through to Middlesborough but before Newcastle station was remodelled for electrification around 1990 there were not so many trains that ran through to the Tyne Valley line, if any. There were only 3 or 4 through platforms and the island platform furthest from the entrance was added around that time. It greatly improved the flexibility of the services. I think the platforms at Felling and Pelaw went after the metro opened and Gateshead East may have survived into the 80s. In 1989 the platforms were still there but the station was closed. Trains were mostly Metro Cammell DMUs with no loco hauled services except football specials to Seaburn.
In the 85/86 timetable there were 3 trains per hour, leaving Newcastle at 20 minute intervals. '15 past was Heworth and fast to Sunderland then all stations (except Greatham) to Middlesbrough.
The '35 and '55 were all Stations to Sunderland. In the evening, the last trains beyond Sunderland were 19h15 then 21h45, with a 22h35 Sa. only to Seaham. Nothing went across Newcastle Central to the Tyne Valley line. Sundays was hourly, all stations, to Sunderland extended hourly or two hourly beyond Sunderland all stations (except Greatham) to M'boro.
On Mo.-Sa. there was a 05h35 Newcastle-Kings Cross '125' which only stopped at Sunderland, Hartlepool, Stockton, Middlesbrough, Northallerton, York and Stevenage.
The Winter 62/63 timetable was 2 trains per hour from Newcastle: '13 past was Gateshead and Seaburn to Sunderland then Seaham, Hartlepool, Billingham, Stockton, M'boro; the '45 past was all Stations incl. Felling, Pelaw and Monkwearmouth to Sunderland. There was a few extra trains in the peaks.
The stations at Easington, Horden and Blackhall Colliery were infrequently served by Sunderland starting trains with a couple of peak exceptions through from Newcastle. (including an 05h52 Newcastle to Guisborough arr 07h58 !)