I get good sound isolation from hf5s: http://www.etymotic.com/ephp/hf5.html -- they are basically ear plugs with the ability to pump in audio. They work well enough to quiet (but not silence) a room/environment/train without audio hooked up.

The other extreme is to just use shooting range ear protection: http://www.earplugstore.com/hunting-and-shooting-ear-plugs.h... (or amazon'd: http://www.amazon.com/Best-Sellers-Sports-Outdoors-Hunting-E...)

I would expect ear protection (to avoid damage caused by extreme pressure changes) and volume reduction to be quite different.

http://www.wisegeek.com/what-are-safety-ear-plugs.htm

I may be misunderstanding you, but you may want to opt for a noise-canceling headset or IEM + white noise rather than complete silence. The reason is that if you have just quiet, any noise that makes it through is guaranteed to be a distraction (fragile). With white noise, almost all outside interference will just blend into the white noise (robust). (By white noise, I mean any sound that you find easy to ignore. This could be certain songs, ambient sound, or white noise.)