In its initial appearance in Hero of the Rails, a section of the bridge was damaged and could not carry Spencer's weight, causing the section to collapse beneath him. Later, Hiro and Rocky came to his rescue.
It reappeared in the thirteenth season episode, Play Time. The track was bumpy after the repairs, which caused the couplings between Thomas and his coaches to be shaken loose. In Hiro Helps Out, Hiro suggests that Percy take his ducks to the track so they could swim in the marsh.
In the fifteenth season, Gordon and Ferdinand were responsible for the Lion of Sodor landing in the mud after hitting a bump in the track. Later, Gordon cracked the track and he and James ended up in the marsh.
The marsh often either becomes a lake or gets dried up depending how much rain it collects.