Talented and versatile Mr. Meggs (Aria, Saturday Games) has based this buoyant, if overextended, novel on his grandfather's real-life adventure in 1916: being conductor (at age 18!) on the special 39-car train that carried the 12th Cavalry Regiment (300 men, 270 horses) from Fort Meade, S.D., to General Pershing's war against Pancho Villa on the Mexican border. And Meggs has lovingly turned his grandfather into a highly appealing young hero: Cassie McGill, a once-promising baseball rookie (till he busted his elbow) with Irish-Catholic virtues, poor but loving folks (a literary Ma, a musical Pa), and a heap of problems. Picked to run the war train because no one else wants the assignment, Cassie must deal with: a Colonel who's in a pathological hurry to get to Mexico in time for the fighting; a second-in-command martinet; swinish veteran troopers who install three hookers in the caboose and steal the rosary beads that Ma gave to Cassie; boxcars unfit for transporting horses (many of which die in agony, despite attempts to improvise stalls from rope); and, above all, an unhelpful ""helper"" in Rocks McAuliffe--Cassie's boozy, ne'er-do-well childhood chum, who's the only one willing to take the thankless ""helper"" job. Furthermore, there's extra pressure on Cassie when a special, sumptuous car is added on to the train en route--that of Otis Webster, a munitions magnate who offers big bribes to Cassie (who's breathless over Webster's daughter) in exchange for speed (he too is rabid to hook up with Pershing). So Cassie pushes the pace as best he can, though precious hours are lost while waiting for a relief crew in Dalhart, Texas (thanks to Cassie's telegraph-message foulup). And even worse trouble arises when Rocks insists on rescuing one of the hookers from trooper villain Snivey: there's a tense, all-cheating poker game (Cassie's a cardsharp), lots of hiding and chasing, and the secret shooting of Snivey (Rocks saves Cassie's life). Unfortunately, however, this dandy blend of action and character peaks early on--and a contrived, stretched-out feeling intrudes after the train gets to New Mexico: Cassie is beaten up for not having made better time; Rocks is illegally court-martialed for Snivey's killing; Cassie rescues Rocks, meets Damon Runyon, loses his virginity (as well as his cash); and there's a final Indian battle, with Cassie as surprise hero. All in all, then, this seems about 100 pages too long and just a bit too incident-packed for credibility. But at its best (especially in the sentimental opening chapters) it's a grand update on the classic boys'-adventure-book genre--textured with phenomenal detail, alive with knowing humor, and often infused with the folk-smart spirit of early Mark Twain.