The fabled Southern Tier is originally an Erie RR line running between Suffern, NY to western NY state. It then became an EL route, then Conrail, and finally an NS route. With each successive owner less and less traffic was routed over it. Trackage rights have been granted to other railroads over the years, most notably the D&H/Guilford and NYS&W.
As for current operations I'm not sure of what is happening west of Binghamton, NY but east of there the line is owned by NS but there is very little NS traffic running these days. It gets slightly better from Port Jervis, NY east to Suffern, NY. Here, the line is still owned by NS but is leased to Metro-North and dispatched by NJ Transit. Having worked on the NJT Mainline dispatch desk during 2002, which covered the Suffern-Port Jervis section, let me fill you in on what you might expect to see. Keep in mind that the freight train information is based on what we did in 2002 and may have changed since then (though not by much based on what I still hear on my scanner).
Obviously the commuter trains are the most numerous. Starting from around 5am through the morning rush there is a train about every hour from Port Jervis. There is a midday lull, except for Train 60-due out of Port Jervis at 1235pm-until Train 53 kicks off the afternoon rush out of Hoboken.
In regard to freight trains NS runs a daily local out of Campbell Hall, NY. HO-8/HO-9 would come out midmorning, usually around 10am and would usually work both west to Port Jervis, 4 Story Jct, and then east all the way to Harriman. They would usually follow 53 west in the afternoon. There was also a night job that switched Campbell Hall. We'd hear them switching the yard, but I don't recall them ever coming out to do any road work.
On the very east end of the Tier, H-55 would come out of Suffern Yard, again around 10am, and work the Main and Bergen County lines as well as switching the industries in and around Paterson, NJ. They would return to Suffern during the late afternoon. The night job out of Suffern is H-56, also working the Main and Bergen County lines.
The NYS&W operates on the east end of the Tier between Hudson Jct. and Port Jervis. Their Southern Division mainline ends in Warwick, NY and they run on the NS Hudson Secondary Track (former L&HR) from Warwick to Hudson Jct. and then the Tier from Hudson Jct. to Binghamton in order to reach their Northern Division. SU-99 is the westbound symbol, SU-100 is the eastbound symbol. They seemed to run usually at night or in the very early morning hours.
Hope this helps, and if anyone can update or correct the information here, please update it.
Long Live The EL :D