Question

Gravel, crushed rock, cinder—these are used as ballast on railroad beds.

correct
incorrect

Answer 1

Correct.

The rule in this case is that it sets apart this set of words as an introduction to the sentence. Dashes also may be used to set apart as a conclusion. See the reverse:

These are used as ballast on railroad beds - gravel, crush rock, cinder.

Answer 2

Answer: the sentence is correct.

Explanation: Dashes, like commas, semicolons, colons, ellipses, and parentheses, indicate more emphasis, an interruption, or an abrupt change of thought. Experienced writers know that these marks are not undifferentiated. Note how dashes subtly change the tone of the following sentences:

You are the friend, the only friend, who offered to help me.

You are the friend—the only friend—who offered to help me.

I pay the bills; she has all the fun.

I pay the bills—she has all the fun.

I wish you would … oh, never mind.

I wish you would—oh, never mind.