Snow Hill to Wolverhampton shouldve closed in 1967, when Snow Hill was closed to main line traffic. But the local service was still quite busy, still running every hour from early morning til late at night with rush hour extras, so closure was refused. So, British Rail replaced that service with just 6 trains per day each way, in rush hours only, with the next timetable change. This meant passenger numbers fell dramatically, so the 3 car dmus were replaced with single cars. They applied again to close it, but again, closure was refused due to passenger numbers. Next came unstaffing and demolition of the station buildings, leaving some platforms with no shelter. In the case of Hockley and Soho & Winson Green, no lighting or name boards either. So, passenger numbers fell again, by now only a fraction of just 5 years earlier, and this time when British Rail applied for closure, it was granted. It had gone from being a busy suburban service, which the stations were quite well placed to the towns they served to an uninviting service, that was useless to many. During the winter months, the stations had become dark and menacing. No surprise really that passengers deserted it in droves.
This (and subsequent responses) rather skates over the political background.
The first round of 'Grant Aid' under the Transport Act 1968 was announced in November 1968, by the then Minister of Transport, Richard Marsh. This included grants for Snow Hill to both Wolverhampton Low Level and Langley Green. (This announcement included the fact that grant would NOT be forthcoming for lines like Skipton-Colne, Okehampton, Glenfarg, Caernarvon, Fleetwood, Cambridge-St Ives, etc, which could be progressed for closure.)
Obviously BR was expected to make maximum economies on lines that were staying open, hence thing like the Surplus Track Capacity [rapid elimination thereof] Grants and conductor guard working from de-staffed stations as widely introduced elsewhere on the network.
This was only a temporary situation as the Passenger Transport Authorities, also established under the 1968 Act became responsible for public transport planning in their areas. So far as the West Midlands were concerned it became immediately obvious that the only priority was unifying the municipal bus operations of Birmingham, West Bromwich, Wolverhampton and Walsall with a focus on driver-only, exact fare only, front entrance diesel buses. The new '79' Birmingham-Wolverhampton bus route through the area served by the line was the flagship demonstration of integration.
The new PTE decided very early on not to subsidise the Snow Hill lines, hence BR had no alternative but to propose closure, which was approved.
(As a resident in the West Midlands at the time and occasional user of the residual services it was very obvious that the nature of the area was changing dramatically. Slum clearance, rapidly rising car ownership, construction of the M5/M6, the decline of traditional metal-bashing industries, a surge in the popularity of vandalism as a youth hobby and the arrival of a new demographic that had no tradition of rail commuting all combined to depress the services’ prospects.)