GR translation: On the beam

[Mike Jing]

V679.7-14   “Uh, Myrtle . . .” Slothrop approaches gold-snooded M.M. a
little deferent, “uh, do you think you could . . .” Jesus they run through
this same routine every time—doesn’t Myrtle wish Sniveling Slothrop would
cut this wishy-washy malarkey ’n’ be a man fer once! She lights a
cigarette, lets it droop from one comer of her mouth, juts out the opposite
hip and sighs, “On the beam,” exasperated already with this creep—
       And Los! the miracle is done, they’re now zipping along the
corridor-streets of the Raketen-Stadt like some long-necked sea monster.

What does "On the beam" mean here?
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