Yes, by a band of volunteers called the deviationists. The diversion required building the Dduallt spiral in order to gain the height needed to avoid the reservoir level.
https://www.2022.festrail.co.uk/__oldstuff__/fr_history_4.htm Part 4 - The Great Deviation
In 1970 an alternative route to the west of the Tanygrisiau reservoir was agreed and the Dduallt spiral, required to raise the line was completed in 1971. In the same year the crucial legal battle for compensation, which had been going on since the 1950s (!), culminated in a hearing at which the Company was awarded £106,000 for loss of profits. But much more was needed of both money and resources to get to Blaenau Ffestiniog.
First there was a new Moelwyn Tunnel, completed in 1977, which allowed trains to run as far as Llyn Ystradau, just short of the new power station. Then bridges had to be built over the four power station water pipes to reach Tanygrisiau and get back on the old track bed.
On 24 June 1978 the opening of the deviation between Dduallt and Tanygrisiau was celebrated with speeches, a party and a 'golden spike' ceremony. The 'seemingly impossible' had been achieved.
Only one mile of track remained to be restored to bring trains back to Blaenau Ffestiniog, but there were still many problems. The rock face just beyond Tanygrisiau was unstable and a serious rock fall demanded costly repairs when money was tighter than ever.
Meanwhile, it had been generally agreed that Blaenau Ffestiniog would benefit from a joint British Rail/Ffestiniog Railway station near the town centre. By autumn 1977, Gwynedd County Council adopted a scheme to allow the Ffestiniog Railway access to the centre. Without financial support at national level, and internationally from the European Economic Community, the work could not have been completed as early as 1982. As with the Deviation, the slog back to Blaenau became a joint effort of Company, volunteers, engineering contractors and labour provided under a Manpower Services Commission scheme. The Deviation organisation was reshaped and 'Project Blaenau' was launched in July 1980 to coordinate the volunteer share of the work.
Work was pushed ahead despite appalling weather to reach the opening date, 25 May 1982, the 150th Anniversary almost to the day of the Company's first Act of Parliament. This was the day towards which all efforts since 1951 had ultimately been directed; the Ffestiniog Railway once again ran from Porthmadog to Blaenau Ffestiniog. The Rt. Hon. George Thomas M.P., then Speaker of the House of Commons, officially opened the station at Blaenau Ffestiniog on 30 April 1983.
