From the [Official Rules of Monopoly][1] on the Hasbro web site under heading *Mortgages* (my **emphasis**):

> No rent can be collected on mortgaged properties **or utilities**, but rent can be collected on unmortgaged properties in the same group.

To me this is unequivocal - no payment, whether or not denoted as *rent*, is ever collected on a property that is face down due to being mortgaged. Not carefully the wording "*in the same group*", and that it does **not** say 
"*in the same colour group*"; thus clearly indicating an applicability to railroads as well as coloured properties.

  [1]: http://www.hasbro.com/common/instruct/00009.pdf