What does "one-track" mean?

having a single set of lines so that trains can run only in one direction at a time; single-track

One-track

adj

adj ·Rare ·Advanced level

Adjective

1
having a single set of lines so that trains can run only in one direction at a time; single-track not-comparable
2
limited to one idea or thought not-comparable

"I have a one-track mind. All I can think about is skiing."