The real question is do you just want to spend 1/2 a day chasing or a full day? If a full day start in Greenville in the early morning and wait to see what moves. The first crew is usually called at 6 but they weren't called till 8 the other day. They will either take empties from Greenville or will head to Branchton for empties to take north. Then they will turn on a ore train to take south with a recrew between 12 and 15:00 at Kremis. Second crew is normally called for 13:00 to head north for more ore or maybe some misc. work if no ore. They usually get a recrew but well after dark.
If a just 1/2 a day then you will have to just judge what you rather do.