IN a full-page program note, the composer Andrew Lloyd Webber modestly explains that he conceived his new musical, ''Starlight Express,'' as an entertainment ''event'' for children who love trains. Over two numbing hours later, you may find yourself wondering exactly whose children he has in mind. A confusing jamboree of piercing noise, routine roller-skating, misogyny and Orwellian special effects, ''Starlight Express'' is the perfect gift for the kid who has everything except parents.

The high-tech scenic environment, designed by John Napier, is something to see, if only at intermission. A three-level Erector set of simulated train tracks, the design recalls a previous Gershwin Theater occupant, ''Sweeney Todd'' - had that production been dipped in pink and purple bubble gum. The principal feature of Mr. Napier's hydraulic construction is a huge steel-and-Plexiglas suspension bridge that twirls and lowers, flashing like a large disco chandelier. Beware of that bridge. Its descent usually indicates that it's time for the cast to skate around in circles until theatergoers of all ages surrender their appetite for a post-performance snack.

According to Richard Stilgoe's libretto, these bouts of skating are ''heats'' in a ''great race'' to determine ''the fastest locomotive in the world.'' The prize is the least costly prop on stage - a silver dollar. But who are the competitors? At first, the audience is introduced to rival trains from various nations (each presented according to ethnic stereotype) -and yet those elaborate identifications are soon forgotten as we're asked to concentrate on Rusty (a steam train), Electra (an electric train) and Greaseball (a diesel). The rules of the race are equally baffling, and so are the arbitrary, anticlimactic results. As chaotically choreographed by Arlene Phillips, the heats of ''Starlight Express'' make television's old roller derbies seem as orderly as a Rockettes kick line.

In the London production of ''Starlight,'' there is at least the novelty of being surrounded by the skaters, who circle the entire auditorium. In New York, the tracks extend only a few rows in front of the proscenium; for most of the audience, the experience is about as involving as standing on the sidelines while other people take one of the lesser rides at Disneyland. There have been other counterproductive alterations in ''Starlight Express'' for Broadway as well. The show has been ''Americanized'' by the plastering of signs like ''Kalamazoo'' and ''Cincinnati'' on Mr. Napier's set. There are some new musical arrangements, which, along with the amplification, succeed in making a theatrical extravaganza sound as if it were piped in from a Far Rockaway junior prom.