Question で

Re: いくつかの路線では、線路の脇で荷物検査があります。
There is a luggage check at trackside for some trains.

I'm not sure why で is used instead of に in both cases above. Perhaps the former means "for" as the translation says. I have taken it as "along the route" for 路線で which indicates location. The latter refers to where the 荷物検査 is. I would certainly expect "~に荷物検査があります.

"To express existence" is not the only one meaning of ある. It can express actions like "to happen/occur", "to be held", etc., therefore で is used to indicate the location of action for these meanings.
e.g.
昨日駅前で交通事故があった。
明日会社で会議がある。
来年東京でオリンピックがある。