I don't know if that train ran in the 70s (probably did), but it's unlikely to have run via Ealing Broadway and even more unlikely to have called there.Was that by any chance the service that came in from the Chiltern lines (via Haddenham & Thame Parkway) and returned in the evening? I remember seeing the service on the maps and timetables, but I'm sorry I don't recall where it stopped. That was probably later than the 70s though (NSE period)
I think the service referred to ran via Oxford rather than Greenford - whether it called at Ealing Broadway - pass.
In the 1974 / 5 timetable, the 0805 (Mon - Fri) Paddington - Worcester (Shrub Hill) via Reading and Birmingham New St is shown as 'also stops at Ealing Broadway 0813 and Slough 0826' (this arrived Reading 0846 so it would fit the idea about being of use to office workers commuting to Reading)
Likewise on Saturdays the 0805 (Sat) is 'also stops at Ealing Broadway at 0813' and on Sundays the 0825 Paddington - Birmingham stops at Ealing Broadway 0833.
No weekday up working shown, but the 2040 (Sundays) Birmingham - Paddington called at Ealing Broadway 2310.
Obviously the public timetable does not show what haulage / stock was involved. The 0805 ex Paddington is shown as 'buffet service' on Mon - Fri.
...a service only stopping at Ealing and Slough, and running against the peak flow (not this line had anything like the commuter traffic that it has nowadays) might well have been on the Mains. In the 1970s they often had the last of the Westerns and Hymeks, later Class 47 became the norm. The stock for these services, and for the Birmingham via Oxford and for the Worcester services, was all in a common pool in Western Region days, and there were a number of half-length sets which were added to services from Worcester etc at Oxford.