Can you pick up a locomotive if it's revealed as a replacement card?
No, once a card has been revealed as a locomotive, you have to "spend" two draws to pick it up, which means you can't do so since you've already drawn one of your two cards.

From the official rules (PDF), emphasis mine:


  If a Locomotive card is one of the five face-up cards, the player who draws it may only draw one card, instead of two. If, after having drawn one card the replacement card is a locomotive, the player cannot take it.