for all I know one of them will invent a cure for cancer in the future

What are the actual odds of that, though? Compared to the good you do (you're on LW, so I'm guessing you're more likely to do some rational altruism and save more than five lives than they are.)

I also note that, if my wife or daughter was one of the people tied to the track, I would unhesitatingly throw myself off. This makes me conclude that I should want to throw myself off the bridge (because the supposedly, flimsily 'rational atruistic' reason -- that I have the potential to help people -- is revealed to be bogus).

I would assume you're massively biased/emotionally compromised with regards to that scenario, just for evopsych reasons. So I'd be iffy about using that as a yard stick.

That said, you also presumably know them better, so there's the risk that you're treating the five victims as faceless NPCs.

I still wonder, however, if there is any possible rational reason to not choose to sacrifice oneself in the scenario. I am unable to come up with one.

Ultimately, it comes down the instrumental values. The five get a x5 and also automatically save four net lives, so you would have to be noticeably above average - but I'd say there's enough low-hanging fruit around that that's far from impossible.

After all, it's not like these people are signed up for cryonics.