Including the Trailer header field with responses that don't have Transfer-Encoding: chunked causes some (overly strict?) proxies to drop the response e.g. IBM Bluemix sends back a 502 instead of the response generated by Express.

I think a possible solution here would be to remove the Trailer header field for 304, in addition to the fields currently removed. Here is where that code lives: https://github.com/strongloop/express/blob/f73ff9243006ea010fffdaa748f06df3a5b986e7/lib/response.js#L192