Usually the hardest part with recycling is separating out the materials, especially once you start having components that are potentially toxic. For example, you could have an all metal subway car that has asbestos insulation... now your recycling cost is huge. Probably cheaper now to just bury the whole thing.

Recycling can raise or lower the cost of disposal, depending on construction and materials. Heavy use of adhesives pretty much makes recycling a nightmare, as well as complex mixes of plastics.