Don't forget that the Builder travels to Vancouver, WA, first before heading east along the Columbia River. The schedule has the train leaving Vancouver eastbound at about 5:05. This is all assuming the train is running on time. As the days get longer, there will be more and more daylight to see more of the run. Even after the sun goes down, there is usually some reflection of light on the water, which can help you follow it for a while. Reflection of city lights on the water may also help, but there will be little detail.
The Columbia Gorge, to be exact, is that part of the river that flows through the Cascade Mountains. On the Oregon side, that usually starts just east of Troutdale. On the Washington side, it would probably be about Washougal. The real scenery begins at the Bonneville Dam complex and then glides underneath the Bridge of the Gods. This time of year, it will still be too dark to see this and anything farther east than this--going east. Westbound, however, all of this would be quite visible in the mid-morning hours.