Hi,
I might be giving misleading info here, but others will no doubt correct me if so..
When a service disruption occurs, that seems to indicate that the train can no longer complete its route, rather than just experiencing a "delay". This can be for any number of reasons, and could meant it failed to start it's journey, or more likley, encountered technical problems en route. You should contact an Amtrak agent if you hear of a disruption, as there is no set response. It might result in a train being cancelled just one day, or for several days, eg in the case of flooding or landslide, or it might be that a bus is being provided to work around a minor problem. If you are aboard a train which has a problem, Amtrak will try to work to get you to your ticketed destination.. This might be mean transfering to a bus, or just waiting aboard the train untill the problem is fixed.
If a train is halted en route before you board it, then your choices are often limited, and you can only call Amtrak and ask what, if any, alternatives there are for you, such as re-booking a day later.
It seems a good plan to me to always have a few decent snacks and a drink in your carry on luggage, just in case, too!
Ed