I assume you made a reservation for a roomette or bedroom. When you do, the "rail fare" reverts to the lowest bucket (level). That is the "rail discount" that you see.
Example: The lowest bucket for a specific segment is $75, but coach is now selling for $180. You buy a sleeper, which is actually the coach rail fare plus the accommodation charge. Because with a sleeper, the "rail fare" portion reverts to the lowest bucket, in this case you would see a "rail discount" show for $105! When you originally made the reservation, the "rail fare" bucket (in this example) was $75, so there was no "rail discount" to be shown.