It seems the obvious response is to prove the attack wrong with some real numbers.
The blurb flat out says that "Amtrak loses more than twice as much per passenger (an average of $396) for first class service as compared to coach class service," yet the OP claims first class services subsidize coach. Both statements can't be right, so if you want to argue against their statement you should probably bring some data to support your side.
After all, if they have their facts right, they're making a pretty good argument.
The problem is that $396 per passenger is a really meaningless number. It's sort of like asking me how much money per passenger it costs me to drive my car. Even if I sit and calculate all that out and come up with say $10 per passenger to drive my car. Great! But what does mean? It obviously depends on how many people ride in my car and the distance I go, pretty much messing up my $10 number.
People that ride buses, trains, and planes, always are going different distances. So maybe it's more accurate in measuring ridership and cost in cost per passenger mile. Ask an airline what their per passenger cost is and you'll get an answer in per mile.
So you wanted numbers, and this is just some average numbers. I don't have time to dig through the files on Amtrak's site, but here are some of NARP's site made simple:
Looking at Amtrak's long distance route only, Amtrak takes in an average of 23.8 cents per passenger mile for sleepers. They took in only 12.3 cents per passenger mile for coach. The average fare for sleeping car pax was $241 vs. only $67 for coach. And for 2008, Amtrak took over 635,000 sleeping car passengers vs. 3.5 million coach passengers. Keep in mind that the average sleeping pax travelled twice the distance of coach passengers too. (1014 vs. 546)
So even though, according to these numbers, sleeping car passenger made up only about 15% of all long distance passengers, they make up for almost 40% of the revenue.
So essentially, this senator is asking Amtrak to eliminate 40% of its revenue for the long distance trains. If you want to write to him that is all you have to say.