One option would be to have the entities transfer into a station and use the Wait step to 'hold' them there until something else triggers them to exit with a Fire step. The advantage of having the entity transfer into/out of the station is that the statistics of the station are captured automatically - for example, the number (average/maximum) in the station, the holding time in the station, and the counts of number entered / exited the station. The Transfer and EndTransfer steps would be used to move the entities into/out of the station and the station.contents queue can be used to animate those entities waiting. See the attached example, built using Simio sprint 91.
WaitInStation.spfx