Westbound, from Chicago to Seattle/Portland, the train is scheduled to pass through Glacier in the early evening hours (6-9pm), that is assuming it is on time. During the winter hours, when daylight is in shorter supply, it is usually dark by that time. Similarly, eastbound, which is scheduled for the passage in the morning (7-9am, i.e.), it can still be dark in the morning, but the sun would be coming up and there is the prospect of some nice sunrises over the mountains. During the summer hours, in both directions, sunrise is very early in the morning and sunset late in the evening, you would have better chances to get enough light for viewing the scenery. The same conditions exist for the traverse of the Washington Cascades (though directions are reversed--westbound trains pass through the mountains in early morning; eastbound trains in the evening) and through the Columbia Gorge on the Portland segment.
The reasons for the discrepencies in lighting is the northernly location, which provides for short days and long nights in winter and long days and short nights in the summer--and of course, there is the constant change going from one condition to the other according the season. In the far north, Alaska and Canada (the Land of the Midnight Sun), the sun does not set in June and there is no daylight in December.
Make sure you recheck your schedule and make sure you are reading the right column in the right direction.
The same day/night/light/dark conditions exist for VIA Rail's "The Canadian," Amtrak's Cascadia Corridor trains, the Montreal and Toronto trains, among others. These conditions can be more extreme on the Alaska Railroad between Anchorage and Fairbanks.