We are doing the NIN over a two day period and are breaking up the trip with an overnight in Aurland. We will arrive in Flam in the afternoon and have scheduled to go on the bus to the Stegastein viewpoint and then a music/nibbles event at 6:30 in Flam. Unfortunately, the local Flam/Aurland shuttle bus doesn't run later in the evening and the music program doesn't end until 8pm. Is there reliable taxi service from the Flam railway station in the evening?
Flam is a very tiny place and according to the Google, there may not even be a currently functioning taxi service. I would contact your hotel in Aurland and ask if they have suggestions for a private transfer, but if they come up empty handed you'll probably need to adjust your plans in Flam so you can take the bus back.
Flåm Travel aka Lokal Taxi aka Aurland Taxi
Definitely reserve in advance. You will be competing with many others for the very limited service. I have no idea about reliability.
Personally, I would either forgo the music/nibbles or try to book lodging in Flåm. Availability of rooms in Flåm is constantly changing as travelers cancel and rebook.