Positions come from each railroad's public real-time feed and refresh every 20–60 seconds. Trains marked estimated are placed between stations using predicted arrival times because their feed doesn't report GPS.
Railroads often don't post a delay until well after it starts. Trainsheet watches GPS for trains still at a station after their posted departure, or too far out to make their next stop on time, and adds that delay itself. Those delays are underlined with dots. You can turn this off above.