Rail-to-trail bike paths are ideal because there is already existing right-of-way (ROW) due to the now-defunct railroad. Same deal with bayous; Houston has done a fantastic job of creating MUPs along their lengthy bayous (White Oak and Braes stand out in particular).

However, it is interesting to not that when many of the railroads were first built in the mid and late 1800's, they were built by private companies such as Union Pacific, which were granted broad eminent domain rights by the government to seize any private land that stood in the way of the railroads.

I haven't been able to definitely verify if this has been the case with Houston's old railroads, but it is noteworthy that many of Houston's railroad ROWs go straight through historic and older neighborhoods (Heights, Third Ward, Fifth Ward). I would hesitate to say that MUPs such as Columbia Tap did not make any use of eminent domain, when there is a high likelihood that the existing railroad ROWs used eminent domain to construct the railroad in the first place.