I don’t see any benefit to a dry run. What does it get you?
If you succeed you have placed a useless device on a bus, the device is eventually discovered and the authorities are now on the alert for devices placed on the undercarriage of public transportation. So when you finally decide to try to plant your device it is now more difficult because the authorities are looking for your operation.
If you fail (got caught) at best the authorities arrest one of your people and learn at the very least who one of your people is.
At the worst the authorities learn who you are, who your friends are, what your plans are and you go to jail and have accomplished nothing. All of these are likely.
Go big or stay home. Why bother with a dry run when you can for the same risk do the real thing.
A terrorist may do a walk through where they would go through all of the motions of the plan with the exception of actually delivering the device. This would involve the entire team going to the park, carrying the bag which would carry the device, getting on the bus and getting off the bus setting the bag down next to the bus and getting something out of the bag.
This would familiarize the team with the park and the busses to make them more relaxed during the mission and test the waters for the tolerance of the staff and customers for their odd behavior.
Attaching a dummy device to the bus would be counter productive.
It is a distraction. While everyone is looking under busses, you plant the bioagent dispersal unit under the roller coaster, 'dust' the wristbands, contminate the water, whatever.
Terrorism does not require an effective device, just the hint of one is usually sufficient to cause the perceived threat level to increase.
It isn't just about body count, it is about the psychological effect.