While not in a museum, this ex-B&M 0-6-0 looks like she just came out of one! On display in Lowell, MA and part of the NPS, this cosmetic restoration was undertaken by volunteers from the Boston & Maine Historical Society. If you could have seen what she looked like buried in the woods at the H.E. Fletcher quarry you wouldn't believe it was the same engine.

While some of the locomotives mentioned above look pretty good, it must be noted that the appearance factor does not always equate with proper preparation and maintenance. The B&M 0-6-0, for instance, had its bearings destroyed when it was carelessly towed from the quarry to Billerica on its own wheels, without prior polishing and lubrication of the axle journals. One half turn of the wheels can ruin bearings, due to the corrosion on the underside of the journal being ground up into the soft bearing material when the axle turns.

Likewise, any engine with asbestos lagging still present is not properly preserved, as the absorbent material holds water against the boiler and causes corrosion, thinning the boiler plates. And when it is removed (including all piping and the backhead), the boiler jacket should be replaced, with some sort of spacers taking up the thickness of the lagging (PVC pipe is sometimes used on the outer shell).

The smokebox, firebox, ashpan and tender should have all ashes, soot and rust removed, and all openings that might admit water should be sealed.

Glossy paint, a completely outfitted cab and lighting are merely the icing on the cake. And for really long term preservation, some sort of shelter is essential, preferably in a building (I prefer a genuine roundhouse, from which the locomotive once worked). The Railroad Museum of Pennsylvania and ExpoRail are good examples of this. Their locomotives that have been kept inside have not required repainting in over 30 years, but those outside still look a mess, despite frequent repaintings.

As always, the best primer on cosmetic restoration is Dave Conrad's little booklet, available on the ARM website here:

http://members.aol.com/belpaire/steamcm.htm

I wholeheartedly agree that a genuine roundhouse makes an excellent setting for preserved locomotives. However, when you move an engine into a preserved roundhouse, or really any kind of indoor setting (except perhaps for a glass-sided shelter that allows for full viewing from outside), I would consider that to be crossing the line between a park display and a museum. Nothing wrong with that of course, just that this thread was about park displays.