I don't know if I would try to shout down a nazi in an enclosed space by myself, either.

I've approached the person being verbally attacked, asked them if they're all right, showed that they're not alone. I've only done it twice, the person was being sexually harassed in both instances, but I think it's just as useful for verbal attacks of racism, as well as homophobic and transphobia. This https://itstopswithme.humanrights.gov.au/respond-racism page briefly talks about doing things like that. It also mentions assessing the situation before you say something and not "putting yourself at risk" of injury. I think someone threatening physical violence would count as a risky situation. Responding in non-confrontational ways that are less likely to result in escalation that (further) endanger the victim and yourself. It basically suggest engaging with the victim instead of the piece of shit attacking them. I think that's really useful advice and something people should talk about more. Because realistically, not everyone is suited for conformation, confrontation isn't always right answer in these situations either, and there are other options besides confrontation or being a bystander. If we project that there are not other options, I think it just further promotes complete passivity among people who are not prepared to or suited for shouting down a nazi.