There is a very long line in front of the ticket gate, because they are limiting the number of people that can go onto the train platform.
"There is a very long line"=「そこにはとても長い列があります」
"in front of the ticket gate"=「改札口の前に」
"because they are limiting the number of people"=「なぜかというと、彼らは人数制限をしているからです」
"that can go onto the train platform."=「駅のプラットフォームに行ける人達の」