All the railroads have what is known as run through or pool power agreements. In a Run Through agreement. Two or more railroads are participating in a service to move goods from Point A to Point B. Lets say that UP and CSX are Cooperating to move Apples from Washington to Viriginia. Since both railroads are participating in moving the same train to the same point. They may agree to let each other borrow as it were the locomotives that are on that train since the cars they are hauling are going to the same place and its easier to not have to change engines when the cars are interchanged. This explains why you often see UP engines on CSX and CSX engiens on UP.

In a pool agreement, A shipper has some product that needs to be sent from point A to Point B and doesn't care how it gets there. For Example, Ford needs several new Autos moved to California. It Doesnt care who gets them there or how. They just need to get there. In this case since the Railroads involved arent really competing with one another they find it better to Co-operate with one another and pool their resources. They make Cars and Locomotives avialable for the service and help eachother in getting the shipment moved. This is why a train with NS, CSX, UP, and BNSF power on the front (Each Railroad participating in the Pool) May Travel NS-UP one trip. then CSX BNSF the next. and then yet another combination the third.

Then there is a case of Transfer moves where UP might send a train of Cars destined for points on the CN over to the CN yard from the UP Yard. This would most likely have a UP engine on the point.

As for your the sheer number of UP locos you are seeing, I suspect that is from the massive 1250 unit order placed with EMD a few years ago. EMD's Plant is in London ONT. It is not uncommon for EMD to take finished locos out for a spin on some local rail moves to test that everthing is in order before delivering to the customer.

A lot of motive power is not OWNED by the various railroads, but leased from various capital ownership group (Helm Financial and General Electric come to mind).

Equipment that comes off lease early because of business conditions, or is leased directly betwenn railroads (CSX has a motive power shortage and BNSF works to fill it) may not get repainted ... after all, that costs money off the profit, and it's essentially cosmetic.