The term is derived from "railroad" - set of tracks on which carriages can be moved, often in stations these need to be shifted to build/rebuild trains. In legal terms it is used to describe the way someone is being pushed (in most cases gradually by given questions in sequence that leads to a particular outcome)into a position.
It does not necessarily mean undue pressure - such as coercion or blackmail. But judges would still be sensitive to defendants or witnesses being lead this way, and would admit an objection to it.

http://www.answers.com/main/ntquery?s=railroad&gwp=13
To railroad:
1. To rush or push (something) through quickly in order to prevent careful consideration and possible criticism or obstruction: railroad a special-interest bill through Congress.
2. To convict (an accused person) without a fair trial or on trumped-up charges.

A different term

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Note added at 23 hrs (2006-11-23 22:55:18 GMT)
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Scratch the Blackmailed - had a re-think (thanks to some comments). Blackmailed may well be too strong, depending on the context.

The idea is that once you are set on the track, there is only one way to go, and that way is both fast and direct.