"Swing" is a technical term within Musical Theatre, for a cast member who does not have an individual role within the show, however they understudy multiple roles. Generally speaking, swings cover ensemble roles, and the ensemble cover the principal roles. This can lead to a domino effect - if one principal performer is off, up to four understudies can perform as they each move up a slot.
Starlight Express is such a technically demanding show that the understudy system can be quite different, with swings covering principal roles. The German production is particularly good at sharing the information of what swings cover what roles, as it is very likely that the audience for any one performance will see several covers.
The swings are informed one day ahead if they are performing.
Every evening there are swings on standby, in case of sudden illness or injury. Sometimes last minute cast changes are made, e.g. 24th May 15 Sian Velazquez was given 20 mins notice that she was needed as Pearl, or on 15th July 15 Toby Poole went on for Electra, finding out he was needed as the show started.
The female roles follow a rather simple system when it comes to the parts covered. Generally every girl covers Buffy, Ashley and the female Components- Volta, Joule, Wrench. Then the stronger singers also cover Dinah and Pearl.
Since there are more male parts, the system of the male swings is more complex. The base level is covering the National Engines and Components (Krupp and Purse, sometimes Wrench), as well as either the Hip Hoppers or/and Flat-Top or/and Dustin. Stronger singers cover the principal roles of Rusty, Greaseball, Electra, Red Caboose and Poppa, additional to some ensemble roles. Certain links can usually be made due to similar role requirements: A Rusty cover usually also covers Caboose, a Greaseball cover may cover Poppa as well.
Historically, the show has often cast one performer to exclusively cover Poppa, and as such this role is not strictly speaking a swing, but they are included in the category.