Splitting hairs somewhat since Chris' "footway" will be perfectly clear, but "footway" is British English for what others might call "footpath" or "sidewalk". In a street or road tunnel it is little higher than the roadway, so "footway" would be entirely appropriate.

In road tunnels, the roadway will be some height above the bottom of the circular tunnel excavation, i.e. closer to the widest point, and footways will be practically at the same level. In an emergency, people simply get out of their vehicles and leg it.

In rail tunnels (one-way tunnels in particular), however, the track bed will be only slightly higher than the bottom of the tunnel, i.e. where the circular tunnel is narrow. Not only would the tunnel have to be enormous to have room for footways at track level, but it would also be an additional danger in the event of emergency since people would have to climb down some considerable height from wagons before they could leg it. Hence, for both these reasons, rail tunnel "footways" are built up higher (on "haunching"), to a point where the circular tunnel is wider and where people can get down easily, as if getting down onto a platform.

Also, the "pedestrian egress facility" in a tunnel is used rather less than than in a tunnel, so is less a "footway" than in a road tunnel or urban street. In some road tunnels, pedestrians are welcome to walk through. Not so in rail tunnels where the speed and "piston effect" would be too dangerous.

Hence "walkway" is the preferred term for a rail tunnel. In addition, "walkway" has the more "technical" ring of means of access in industrial facilities or other raised access; and in this context it is used by railway service personnel, more so than in a road tunnel where people will drive in and cone off the nearside lane, etc.

Kiwipedia's tunnel lexicon says:
banquette haunching, walkway, kerb; can also be a sidewall kicker (because it resembles a walkway until built on)

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Note added at 11 hrs (2010-08-08 08:52:40 GMT)
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@ Joshua : banquettes are indeed found in sewers, and like railway walkways are built up above the level of the sewer channel (for space reasons and so people's boots don't get overtopped!), but there is no reason to retain "sewer" every time the word comes up.

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Note added at 12 hrs (2010-08-08 10:09:45 GMT)
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"Also, the "pedestrian egress facility" in a rail tunnel is used rather less than than in a roadtunnel"