Hi Colin,
It’s a shame the Postbus trip over the Grimsel, Nufenen, Gotthard and Susten passes won’t be operating in May. You probably realise that is because the passes are closed over winter due to metres-high snow cover, and it takes weeks of work to clear the roads in spring. You might be interested in this 6-minute video showing what’s involved in clearing the Susten Pass road to make it passable and safe again each year:
https://www.youtube.com/watch?v=d91yjs7UdE8
The Rosenlaui bus runs a limited number of services from 17 May 2025, but the frequency increases from 29 May. The online PDF is here:
https://www.oev-info.ch/sites/default/files/fap/2025/pdf/31.164.pdf
This service is also shown in the normal SBB online timetable:
https://www.sbb.ch/en
The Meiringen – Innertkirchen – Engstlenalp bus doesn’t start running again until 14 June 2025.
I don’t know whether this will interest you, but if you are going to Grindelwald, the bus up to Bussalp is a magnificent scenic ride winding up the mountainside, with wonderful views from the top, where there is a restaurant. You can do hikes from there if trail conditions are suitable at the time. May might be a bit early though, as it’s at an altitude of 1800 metres above sea level. There’s a short video here:
https://grindelwald.swiss/en/map/detail/bussalp-94e24abc-1d38-43f9-b255-c391e47f1fd5.html
More info on Bussalp here:
https://www.grindelwaldbus.ch/en/offers_services/offers_summer/data/bussalp_excursion-destination.php
This bus runs from 17 May 2025. You can find it in the normal SBB online timetable, or the PDF file is here:
https://www.oev-info.ch/sites/default/files/fap/2025/pdf/31.126.pdf
If you have any follow-up questions, please specify the exact dates in May that you will be visiting, as May is a time of transition in the alps, and knowing your dates will help us to avoid making suggestions that won’t be possible during your stay.
I hope you will be able to come back some time to do the bus trip over the four passes of the Central Alps. I have done it twice, in September 2016 and again in September this year, and it still ranks as one of my all-time favourite things to do in Switzerland. I also did the trip over 3 passes in 2019, which includes the Furka Pass. That was wonderful too, but the logistics aren’t as straightforward, as you have to change buses.
I hope this helps!
Alpenrose