When you make your reservation make sure to tell the agent your husband is using a walker and will need help with boarding and probably red cap assistance, that way it will be noted and your tickets and on SCA 's and Conductor's rooster.
Whats the issue here? No issue at all. Your rights are clearly spelled out in our Constitution. Amtrak would have a lengthy court case on there hands it I was thrown off the train If I refused a search without reasonable cause or a warrent!