Why was the bridge already down when the second train arrived? In the first episode of the new anime "Koutetsujou-no-Kabaneri", the first train had to stop before they let it enter the station.
(which can be seen in the following picture)



Why was the bridge already down before the second train even arrived? This made it easy for the Kabane to invade the station.
(here you can see that the bridge was already down before the train even arrived)



Was this an error? Or is it a part of the plot, to let the Kabane enter the station?
It's not clear as of now why Koutetsujou (the first train) had to wait before entering the station whereas Fusoujou (the second train) didn't. 

However, a likely reason is that they were expecting Fusoujou to arrive on that day whereas Koutetsujou was to arrive on the next day, but was running ahead of its schedule.

Moreover, I don't think they expected the whole train to be filled with Kabane. They most likely only expected a few to be bitten and planned to get rid of them at the station, like they did with Koutetsujou.


  Why was the bridge already down before the train even arrived?


It wasn't. The bridge was lowered when they saw Husoujyou coming, as can be seen here: