West’s play, first performed at Seattle Repertory Theatre, doesn’t avoid the pitfalls. She stacks the emotional deck with a sentimental plot that suggests audiences can’t abide nuance. If it’s not enough that the train’s staff members have to remind us again and again how ungrateful the unseen white passengers are, West supplies a drunk, racist, sexually violent white conductor (Richard Ziman) so cartoonishly evil he should be twirling a mustache while lashing a damsel to the railroad tracks. (A second Caucasian character, a stowaway played by Emily Chisholm, is a tad more sympathetic, if you don’t count the vile epithet she throws in the face of Butler’s Sister Juba.)

You wait out the rather haphazard plot entanglements, the jokes and solemn pronouncements by the broadly sketched characters to get to the songs, all of which are handled with an assured showmanship, owing in part to Sonia Dawkins’s polished musical staging. Marshall, as an elder porter and head of a family of Pullman employees, moves with debonair grace. And Butler, the flinty Aunt Eller in Arena’s smash revival of “Oklahoma!” as well as the veteran actress of last year’s backstage drama “Trouble in Mind,” here brings a crowd-pleasing carnality to the role of a brassy singer of the blues.

A quarter of a century ago, Derricks won a Tony for his portrayal of a washed-up legend of soul in the original production of “Dreamgirls” (which happens to be playing in a terrific revival across the river at Signature Theatre). It’s a pleasure seeing him onstage again, this time imbuing a character with far more magnetism than it might otherwise radiate.

Derricks’s Sylvester is the son of Marshall’s Monroe and the father of Warner Miller’s Cephas, three generations who, in the summer of 1937, find themselves working side by side on a Panama Limited from Chicago to New Orleans. To the chagrin of Monroe, a kindly company man, and Sylvester, an organizer for the porters’ union, Cephas wavers over whether to return for a sophomore year at the University of Chicago.

The generational struggle might be a sleek and elegant vehicle for the songs. But the story is weighted down with time-sucking subplots. Although it’s set on the day boxer Joe Louis took the heavyweight title from Jim “Cinderella Man” Braddock,” the victory is never translated into quite the watershed moment for the porters that one anticipates. Too many other events get in the way: the disclosing of the murky romantic past between Sylvester and Juba, the friendship of Cephas and the backward stowaway, the ham-handed scheming by Ziman’s Tex to undermine the union man. For a play set on tracks, the story is rolled out without a lot of sense of where it’s going.