A SPAD is committed when someone crosses a Danger (red) signal, hence the name. SPADers tend to do this when they either are distracted or when they are going at full speed past a Caution (yellow) signal, or just attempting to skip a station when a signaller is on duty.
The game is designed to detect a SPAD and apply the offending train's emergency brake for twenty seconds. If too many signalls are passed at danger, the train will automatically be deleted and the service will be terminated, so they cannot complete their route. This prevents the offender from gaining experience points. If a SPAD incident occurs during a Qualified Driver training, the trainee will automatically fail.
Also, when a Signaller or a High Rank is in the game server, they may delete trains that are driving poorly. People who keep SPADing and creating a major traffic disruption may be kicked out of the game server.
Users who SPAD typically show other signs of poor driving. Signs of poor driving include SPADding, skipping stations, going at max speed on a crowded line, and so on.
A SPAR is similar to a SPAD, but it is not the driver’s fault. Due to an oversight on the Signaller’s part or a Signaller trolling someone on the line, they set the signal to red and the train can’t slow down in time, so they SPAD. A Signaller can be held solely responsible, but SPARs may also occur due to technical issues, such as a script error.