1st Class vs. 3rd Class

Can someone explain to me what the difference was between 1st class trains and 3rd class trains?

I have some old time tables from the RRRR, and the older ones from 1912 even list a 2nd class train?


http://www.raritanriver-rr.com/TT04281930.htm

Generally speaking, 1st class = passenger train; 3rd class = freight train.

Most RR which had 3rd class trains in their employee timetables did so that the trains were 'scheduled' and therefore didn't need trainorders or an operator to write them. Pretty much eclipsed today by TCS, form D's, etc.

First class are passenger and mail trains
Second class any general mainline haul.
Third is local switchers etc. the class of service guaranteed what trains got priority by dispatchers and tower operators.
Todays railroads do no longer go by class of service so a Amtrak may sit in siding while mainline tonnage flies by.

Some roads at some times used "2nd class" for mixed trains ... or mail trains ... or whatever! I've seen timetables with 4th class trains, and not all timetables used all the consecutive class numbers at the same time. In the interval between Amtrak and the UP absorption, Chicago & North Western tried first class freight trains, possibly piggyback, to speed the service.
Railroad publicity sometimes referred to "high class, 1st class, extra fare express passenger trains", like the "20th Century Limited". Did the cheaper through passenger trains appear in the employee timetables as 2nd class?