The way valve does it in doomsday event is, that there are 3 minigames that you teleport to and at the same time, the game automatically puts you into a cage and lets you go. You need entity tf_logic_minigames that you name to lets say minigames and then you trigger the minigame upon any time, like point capture like "OnTrigger minigames TeleportToMinigame 0" (or replace 0 with 1 or 2 for different minigame). You need spawns for the carts too where they spawn and so forth. This should get you started. Look doomsday_event for more information, dont look Gravestone (pl_rumble_event). It works more special way and may just confuse you more.