The idea in the first panel is that the players go along with the adventure hook to a point, effectively starting an adventure, then they can do whatever.

The "herding cats" version of this would be putting the hook out there, and the players potentially not even realizing it is a hook. The game master would set up the start of everything, potentially hint heavily at the mysterious hooded figure in the corner of the tavern, and then the players run outside, steal horses from the stable, and ride off into the mountains instead.

An extreme example, but my point is that they're at LEAST talking the to the mysterious figure in Ellen's version before stealing any horses.

Rails

When Ellen says "off the rails", she's referring to the players having the freedom to do what they want, and not being forced down one path. Railroading would be the opposite of that. As such, she's suggesting being on rails to get things going, and then letting the train (adventure) fall where it may after that.