Hi Asanka and Vish -
Rockoyster is, as usual, succinct, and correct. If you really want to take transport up into the high mountains April, some form of transportation will often, but not always, be available for each specific place. If you decide on some specific places you can search for the names, and thus schedules for maintenance shutdowns, in a few ways.
You can search in MySwiss Alps, which provides dates when services are shut down:
You can search by name on the Internet....for example, Schilthorn, which will lead you to their site were you can find have information on transport availabilities, timing, and options
You can use the SBB timetable and check a specific date for travel between wto sites...such as Interlaken Ost (IO) and Lauterbrunnen. IO is not on a high mountain....it s between mountains.
That page gives essential tips - check it out because it will be useful whenever you travel - and links you to the SBB website:
Best of all is to pick a time of year when you can have nice weather, easy travel around the country...which is scenic almost everywhere... snow on the higher mountains, and clear easily accessible places to walk among and between the higher mountains on trails ranging from really easy even for children, and fairly level, to very hard.
That means between mid-to-late June and late September....even early October.
Late June and September avoid a lot of the high tourist density of July and August.
September is the month which, on average, has the best weather.
Since my own interest are in both the lower altitude countryside and also the high Alps, I travel in two seasons.
1.- Late May or early June for country regions such as the Emmental, while skipping the Alps.
2.- Then, in early to mid-September, the Alps, with some time in the lower altitude regions.
I do avoid the Jungfrau Region during weekend of the Jungfrau Marathon, typically around September 7th.