In the Milwaukee area, you are likely to see trains from 5 different railroads. Canadian Pacific runs on the tracks that were previously the Milwaukee Road and Soo Line. Canadian National runs on the tracks that were previously Soo Line and Wisconsin Central. Union Pacific runs the tracks that were originally Chicago & Northwestern. Wisconsin Southern runs on tracks that were originally several different railroads and are now owned by the state. Amtrak runs trains on Canadian Pacific's rails.
Waterford Avenue is a grade crossing on Canadian Pacific's C&M subdivision. It's a good place to see Amtrak trains, with Hiawatha and Empire Builder adding up to as many as 16 trains each day.
Duplainville is a spot near the crossing of Canadian Pacific's Watertown subdivision and Canadian National's Waukesha subdivision. In addition to the CP and CN traffic, you can see Amtrak's Empire Builder on the CP and the occaisional Wisconsin Southern train using trackage rights on the CN.