What is a word that means "to railroad the conversation"?
You're probably looking for digress, to deviate or wander away from the main topic or purpose in speaking or writing; depart from the principal line of argument, plot, study, etc.
There are various other words with much the same meaning - meander, wander, deviate, sidetrack, ramble, etc., but none are so tightly associated with discourse (particularly in speech).
One option is deviating:
v. de·vi·at·ed, de·vi·at·ing, de·vi·ates
v.intr.
To turn aside from a course or way.
To depart, as from a norm, purpose, or subject; stray. See Synonyms at swerve.
v.tr.
To cause to turn aside or differ.
Another option, to continue your metaphor of railroading a conversation, is derailing:
intr. & tr.v. de·railed, de·rail·ing, de·rails
To run or cause to run off the rails.
To come or bring to a sudden halt: