A code phrase used by the SkyTrain officials (for the most part, Skytrain control) to indicate an emergency alarm which may indicate an unauthorized entry into the guideway area. If a yellow tango alarm is tripped while a train is incoming or outgoing, the train will emergency brake). As far as I know, a yellow tango is only applicable for stations. All yellow tango alerts have a code word suffixed to the end, indicating the type of alarm that has been tripped:

yellow tango pie strip - Someone or something has entered the guideway area and set off the guideway sensor alarm. The origin of 'pie strip' refers to the long, red grill-like plate set on the track that acts as the intrusion sensor. Most commonly, this alarm is tripped after someone throws in a pop can or bottle. Less commonly, people realize that they're on the wrong side of a station on a dual-platformed station (i.e. New Westminster Station, King George Station), and think it's a smart idea to run across both tracks to the other side. Very rarely is a 'pie strip' alert triggered because of someone who has entered the track with an intention to harm themselves. Also shortened as 'pies'.

yellow tango gimms - This is either one of two alerts. One possibility is that 'gimms' is a codeword for 'gate', and is therefore triggered when there is an unauthorized opening of a outside walkway gate. Another possibility is that it pertains to the pressing of one of the emergency stop buttons located in the station emergency cabinets. These buttons, as far as I can tell, are incredibly sensitive. In fact, the most common reason for an emergency stop to be triggered is because someone was leaning on the emergency cabinet.

yellow tango zulu - I believe that this codeword really stands for 'zone', in which the zone sensors at the train entrypoints were tripped. If you pay attention to the sides of the stations, you will notice a metallic 'bar' with holes in it, and then another bar opposite to the larger one. These are sensors of some kind, and any time one is tripped illegally, a zulu alarm is sounded. Zulu alarms are temporarily disabled when an authorized train enters, and, when they are disabled manually by way of a key.

A yellow tango alarm will halt the oncoming train as fast as possible, dependant on how close it is to the alarmed station. A train which is a good distance away will come to a calm stop, while a train which is mere meters away from the station entrance will stop almost instantly. (SkyTrain linear inductor technology allows the train to go from 80km/h to 0km/h in not even a second, but would naturally cause injury to those on board, and probably destroy the train chassis in some fashion).

On-board intercom and passenger strip alarms do not create yellow tango alerts.