What would have happened if St Pancras station had been closed?

In 1980 the were serious proposals, and not for the first time, to close both Marylebone and St Pancras stations in London.

Jago Hazzard takes us through the plans and why they came to nothing. Before I go any further, let my suggest you like this video, follow him on YouTube and have a look at his Patreon page.

One thing he does not explain is how trains from the Midland Main Line would have reached Euston. There was no existing connection they could have used nor any obvious new one that could have been built.

I don't know what the London Amenity & Transport Association had in mind in 1980, but a post on RailUKforums tells us what British Rail had in mind when there were serious plans to close St Pancras in the 1960s:

The broad outline plan was that the Bedford Suburban services would be diverted from St. Pancras into Moorgate, much as it happened later with Thameslink. Main Line services were proposed to be diverted from Market Harborough over the now closed line to Northampton, and into Euston, with the lines involved electrified. It was also proposed at one point that the line between Market Harborough and Bedford would close, with Wellingborough and Kettering losing their rail services. 

For about four years the St. Pancras to Nottingham Newspaper traffic, and the remaining St. Pancras to Glasgow overnight service were diverted to run out of Euston over the Northampton to Market Harborough route, before reverting to St. Pancras around 1970 after the station at St. Pancras had been reprieved.

Several floors of the old station Hotel at St. Pancras, which had been in use as offices, were relocated elsewhere during the period when closure seemed certain. Even after it was reprieved there was a lot of vacant office space left empty, and walking about up there could be very spooky at times.

It's worth reading the whole thread there. Other contributors say Bedford to Market Harborough would have remained open or that only the line between Kettering (Glendon North Junction be be precise) and Market Harborough would have closed.

I like the idea, possible if the latter closure had taken place, of a regular semi-fast service from St Pancras to Leicester via Corby and Melton Mowbray.