Lets say the BNSF and CSX operate a train pair that runs from Los Angeles to Philadelphia, 2000 miles, 6 days on the BNSF and 1000 miles, 3 days on the CSXT. The BNSF puts its power, two 4000 hp engines, on the train out of LA and the CSXT puts its engines, two 4000 hp engines, on the train out of Phillie. After 9 days, they can use the inbound power to power the outbound train.
Railroads calculate engine usage by "horsepower hours", how many horsepower by how long its on another railroad. Each set of CSXT power is on the BNSF for 12 days (6 out, 6 back) so that is 24 hours/day x 12 days x 8000 hp = 2,304,000 hphrs. Each set of BNSF power is on the CSXT for 6 days (3 out 3 back), so that is 24 hrs/day x 6 days x 8000 hp = 1,152,000 hphrs. The BNSF will "owe" the CSXT more hphrs than the CSXT will "owe" the BNSF. To even up the balance the BNSF will add engines to the pool, the BNSF powering 12 of the 18 trains in the cycle and the CSXT powering 6 or the BNSF letting the CSXT use twelve 4000 hp engines for a while.