To get the Springfield Downs your level requirement is level Level 16 and you are also required to have activated the Double Down on She’s a Loser Pt. 1 quest in your task book. The Springfield Down is your all out race track where you can place your bets and win an considerably large sum of money. Unfortunately the Springfield Downs does not and will not let you win donuts. You can consider the Springfield Downs a mini-game where you can place a bet on any dog [for a price] each hour and if you win, you get to double or even triple your money!
The Springfield Downs at first will cost you 38,000 cash and 24 hours to build but once you have completing the building you can begin playing those betting mini-games on it. There is little strategy to winning this race game so it is mostly down to luck, if you find yourself losing more money than you can afford then it is suggested that you stop playing. The Springfield Downs is really there to open more quests for other members and their job list. You will find by activating Springfield Downs you will also gain quests to unlock other characters like Mayor Quimby.