I have experienced similar change-in-exposure sequences like this, too. I am using a Rebel XTi.
Like Allan, I find myself shooting more and more in manual mode. Starting off in shutter-priority at 1/500 if I can (at ISO 400) in any kind of good lighting, I will metering off the ballast, and switch over to manual mode and match the exposure.
I will then take a test shot of the "scene" and see how things look. The histogram is helpful, unless I'm in the forest, where the dark trees will throw things off.
I don't use exposure lock for one reason - I've had it unlock before for some reason or another (like the camera powering-down?). Also, I like to use TV mode to set my initial exposure just in case lighting changes (a loner cloud, for example) as the train approached or I just "chicken out" with manual mode. I can throw it back to TV mode and hope for the best.
Funny, I hardly ever had such exposure challenges when I was shooting my EOS-10s 35mm usually in Shutter-priority with Kodachrome 64!