The coaster runs on blocks. Umm. Blocks are to put it simply, sections of the track that only allow one train on each section. Thats what brake runs are for, to separte blocks. If a train is on a block, as another train aproaches it, it will be stopped by the brake run until the train after it clears that block. After that, the brakes will release, and the stopped train will continue on.
A block error could be a few things. They are caused by computer errors. A bad one would be two trains on the same block. The one that probably happend on Shockwave when you saw it was the computer thought there was a train on the block in front of it, but there wasn't. Maybe it wasn't even an error, but the train was stopped because of a train ahead of it on that block.
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Everybodys got something to hide except for me and my monkey!