Bill and Ben are tank engine twins.
Season 1--- April Fools, Secret Tender Club [do not speak], Edward's Big Mess-Up [cameo], Tricked or Treat, and Percy on the Run [cameo]
Season 2--- Christmas [cameo] and Thomas in Love [Ben only]
Season 3--- Gameshow Carnival 2: Roof-top Stranded [Ben only, Bill cameo]
The twins are likely to appear in Season 3.