So getting back on topic: the 24 hour rule still applies. For example if you're going from MSP to CHI, but you want to stop off in MKE for a night. You can use the multi-city tool to book MSP-MKE-CHI. Select the Empire Builder for your first leg (obviously) then select a Hiawatha train inside 24 hours of the scheduled arrival into MKE. The system will price your trip as if you were only going MSP to CHI!
Next week the fare from MSP to CHI is $58 while the fare on the same train, MSP to MKE is $50. The going rate for MKE to CHI is $25. If you bought the tickets, MSP to MKE then MKE to CHI, you'd have to pay $75. But if you use the multi-city option you'd only pay $58. The Hiawatha is also unreserved, so you can use that second ticket anytime you wish! You do not have to use for the time you selected.
This also works for the Silver Service, but it only works for coach fares it seems. (Once daily service resumes) I just priced out NYP to MIA trip in June for $130 in coach. NYP to SAV on the same day is $114, and SAV to MIA, 22 hours later is $70 making you spend a total of $184. But when you plug it into the multi-city option, you can get a total of $130 with a overnight stop in SAV. Obviously both trains are reserved, so you'd have to take your reserved trip.