short version....engines are operating. Mainline is a bit of a problem between PTC and not being able to use Amtrak's liability umbrella for insurance. Just have to find a friendly railroad (Iowa Interstate is running their QJ 2-10-2 in August/September).
C&O 614....Basically stored 1985-96. Operated 3 years and a total of about 20 trips between Hoboken, NJ and Port Jervis NY 1996-98. Ran out market. Hasn't operated since. Doesn't have anywhere to run, so it sits until a situation presents itself to justify rebuilding it. Display C&O Heritage Center Clifton Forge, VA.
N&W 611....figure you're up to date on this one. Restored 2015. Operated NS 2015-17. Victim of current Amtrak policy, so shuffling basically between NC Transportation Museum and Strasburg Railroad, which seem to be the only places that work for it to run. Very limited options of where you can take an engine like that. Too big for most tourist railroads.
Reading 2102...last operated October 1991. Nearing end of rebuild at Reading & Northern in Pennsylvania. Steam program restarted with 4-6-2 425 in 2007, so it should get a decent amount of use.
Southern 4501....restored to operation 2014. Operated one season of mainline trips on Norfolk Southern in 2015. In operation Tennessee Valley Railroad Museum Chattanooga, TN. Bridge problem is affecting offline trips to Summerville, GA, so future is a bit cloudy.
Frisco 1522....last operated 2002. Group disbanded. On display St. Louis, MO.
Milwaukee 261...operated rather frequently on BNSF through 2016/17. Operates out of Milwaukee area, usually on Twin Cities & Western. Mainline ops affected by Amtrak policy.
Southern Pacific 4449....operated occasional trips on BNSF through 2017. Victim of current Amtrak policy. Based out of Portland, OR.
SP&S 700.....same situation as 4449.
Santa Fe 3751....victim of Amtrak policy. Just finishing rebuild, so good to go for 15 years if they can find a place to run.
I think that about covers it, unless you have a specific loco that I didn't address. Several more engines being rebuilt, so maybe they can find places to run.