With permission from cornelius, I have done an update to ngrailsw.grf. The new version (0.92) is attached.
I had recently been using cornelius's Narrow Gauge rails in OpenTTD while testing Wile E.Coyote’s Serbian Rail Set. There were a few incompatibilities between OpenTTD and the Narrow Gauge rails grf. I made some changes to correct these problems.
I then decided it would be good to have compatibility with PikkaBird's viaduct bridge. I have added this with PikkaBird's permission . Wile E.Coyote’s asked if I could add adjusted rail costs, wallyweb requested the long depots.
Changes since ngrailsw.grf v0.91:
1. Support for PikkaBird’s viaduct bridge (see the readme file).
2. Adjusted rail costs for narrow gauge rails.
3. Narrow gauge rails are now deactivated for the Toyland climate.
4. Optional long depots, enables depots to be placed inline without gaps in-between. Enabled by grf parameter, see the readme file for details.
5. A bug in v0.91; the centre section of a 5-tile long suspension bridges was shown with the wrong track type, now fixed.
6. OpenTTD compatibility: Correct track type is now changed to narrow gauge. Various OpenTTD buttons and cursors added (has no effect for TTDPatch).
Please let me know if you find any problems or have suggestions for further improvements. Note, the DOS version (ngrails.grf) is untested, could someone let me know if it works ok?
My thanks to PikkaBird for permission to use narrow gauge viaduct bridge sprites, based on his original viaduct sprites.
Thanks to Wile E.Coyote and wallyweb for testing.

Edit: Narrow Gauge Rails v0.93
New release posted 20/01/08. See my latest post for details of changes.
Edit: v0.93a bugfix release posted 23/01/08