Up until yesterday, it was possible to earn 1000 Choice points per Enterprise rental. That offer has ended. However, between now and March 31, it is possible to earn 1000 Amtrak Guest Rewards points per Enterprise rental. See details here:

http://www.enterprise.com/car_rental...AMGRBNS-_-null

However, keep in mind that like the Choice offer, it appears as though you cannot use any discount AND receive Amtrak points. So you have to decide if 1000 Amtrak points is worth more than the discount you may have otherwise received.

Keep in mind that 1000 Amtrak points equals a free one-way ticket on some short-haul Amtrak routes.