My quick sampling of ~10 trains came up with an average around 65% on-time during the last year. The only one I've ridden, the California Zephyr, is less than 50%.
http://blog.amtrak.com/2015/02/message-amtrak-regarding-time...
http://trn.trains.com/news/news-wire/2017/03/24-judge-sides-...
http://www.jconline.com/story/money/2017/08/24/csx-digs-its-...