As long as the server can manage it, and (as you said) there are ways to get on the train while in motion so it actually makes a difference, rather than just people seeing a fight through the windows of a train that only takes around ten seconds to pass them. If having it uninstanced takes away a large portion of the fight capabilities and entertainment for relatively little in alternatives, I'll go for instanced. If I can the inside full entertainment and un-instanced fun capabilities, I'll take both. Just looking for the larger total value here.
