-The train could leave Boston.
-The train would leave Boston.
Is there a difference? Are they interchangeable?
I think yes! The difference in meaning lies in the modal verbs used, 'could' and 'would'. Yet I feel that we need more context to better understand the actual meaning of the two sentences.
"Could" is generally about capability "Would" is generally about possibility.
The difference in those two sentences would be:
"The train could leave Boston": the train is capable of leaving Boston, but whether it will or not is uncertain. The sentence simply says that it can, nothing more.
"The train would leave Boston": the train can and will leave Boston, it's simply a matter of when.
As you can see, their meanings are slightly different, and are not interchangeable