Note thsat there are other cases of one railroad using another's tracks other than the trackage rights as defined above. First, there can be jointly owned lines, as certain tracks in the Powder River coal field area, built and owned by both BNSF and UP. Only one railroad has the responsibility for maintenance and dispatching with the other paying a share of the costs. Possibly the Denver - Pueblo line is now in that catagory, although in the past when there were two tracks, one was clearly D&RGW owned and one Colorado-Southern (Burlington System) with joint operation and the Sante Fe having trackage rights. Then there is the case of the B&O's Royal Blue passenger train service from Jersey City to Washington. This was clearly an interline operation, with revenue going to the Central RR of New Jersey, Philadelphia and Reading, and the B&O, but B&O equipment, locomotives, and CREWS, both on the engine and trainmen, ran through. In fact, the transition from B&O to Reading at North Broad and Reading to CNJ at Bound Brook were made without stopping. Other freight and passenger operations are through operations by two or more railroads with crews usually chaning and sometimes locomotives too. Intersting that on the UP, for a while the City of San Francisco ran through with the same locomotives Chicago - Oakland, while the CZ swapped power at both Denver and Salt Lake City. MP power on the Colorado Eagle ran through to Denver over the D&RGW from Pueblo.
Intgerurban and streetcar systems had numerous cases of trackage rights, and one that still exists is the South Shore running on CN or Metra track north of Kensingtonn to Randolph Street, Chicago. They also had joint operations like the Cincinnati and Lake Erie cars running into Detroit.
The Milwaukee streetcar system used a section of track actually owned by the North Shore at one time.
There were two sets of trackage rights involving interurbans and the Chicago Rapid Transit Co.
The North Shore Line had rights on CRT from Linden Ave. in Wilmette south to 63rd & Dorchester, later cut back to Roosevelt Rd. CRT had rights over the North Shore Skokie Valley line between Howard and Dempster Streets.
The Chicago Aurora & Elgin had rights over CRT from Laramie Ave to Wells Street Terminal. The CRT had rights over CA&E from Laramie Ave to the Westchester Branch. CA&E handled no local traffic between any stations served by the CRT, including those on its own tracks; i.e., west of Laramie.
There was an excellent article in TRAINS in the 1970's which explained trackage rights and various other operations where one railroad's trains operated over somebody else's tracks.
Dave and Paul, nice posts, but shouldn't haulage rights also be mentioned and the difference between the two be discussed to completely explain why two (or more) roads are sharing the same track?
Art
Haulage rights are probably analogous to codeshare arrangements among airlines. They are a response to the end of interline rates. Strictly as an example: BNSF has a haulage rights agreement with NS between Kansas City and Detroit for through traffic from west of Kansas City. BNSF will solicit traffic between Los Angeles and Detroit and bill the shippers the going single line rate over BNSF from Los Angeles to Detroit. BNSF contracts with NS to handle the shipments between Kansas City and Detroit in NS trains for an agreed fee. The shippers never have to deal with NS.
Haulage rights are a marketing arrangement, trackage rights are an operating arrangement.
Take a look at this.
http://www.trains.com/trn/default.aspx?c=a&id=273