It's not very simple - some young riders who do speedway also race Dirt Track - on modified MX bikes, on pretty long tracks (typically 600 - 800 metres), modified ovals, always having at least one right hand bend, some are "kidney shaped", some very large "L" shapes.
So a lot of these kids (ie their parents) have several bikes, that they race at different tracks & meetings al around the countryside - for example, if a kid here ni Albury wants to race Junior speedway, they can race at Mildura 600km away, or Moe 500k, Adelaide 1000k, Gosford 650k - as well as Dirt track at home in Albury, Broadford 250k, Wagga 140k, West Wyalong 300k, Temora 250k, Canberra 400k, Griffith 275k, Mildura 600k, Cowra 500k, Newcastle 720k - get the picture? and surprisingly, not many kids do both, more do Dirt Track in the southern part of the nation than speedway.
The huge dedication, effort & expense by families & the very rare sponsor actually has the effect of making our kids very appreciative, dogged, versatile and determined to do well - not only for themselves, but as a show of return for their families and supporters efforts.
I disagree that the Junior scene is well structured - rather, there are hotbeds of dedicated, harworking zealots who love their kids and the sport, and this produces a stream of young blokes who are regularly punching above their weight on the world scene!