In one sentence, describe what the following article is about:

Occasionally, the BART system encounters delays due to reasons ranging from medical emergencies on trains, equipment issues on trains and tracks, and police activity at stations. If it is very critical for you to be somewhere at a particular time, please aim to take a train that is expected to get you to your location well in advance.  If arriving at a particular time is critical, check before entering the system whether there are any system-wide delays. You can use the BART service advisories and also check for announcements displayed at the station. If the BART delay occurs when you are already in a downtown San Francisco BART station and need to go to a location in San Francisco, consider using the Metro system (which shares station locations with the four downtown San Francisco stations). You can also consider using Uber, Lyft, or other car-ordering services. Order the service when you exit the train so that it arrives by the time you are out on the street. However, keep in mind that car-ordering services may experience significant surge pricing during system-wide BART delays, so check the surge multiplier before you use them. Keep in mind that during system delays, stations can get very crowded. This can make them unsafe to navigate. It can also make it harder to get Internet access through your cellphone, because a large number of people stuck in a small geographical area may be trying to use the Internet to while away their time and communicate to their friends and colleagues that that they are stuck, thus clogging the network.
Understand other reasons why delays occur.