There is a bus that goes to the Ohel that is normally filled with Bachurim. The lights are on the whole ride. Can a woman come on the bus herself? With her husband?
There is no prohibition of Yichud when one woman travels with two or more men. At night, or when traveling, three men are required.
An independent issue is whether it is altogether appropriate to have women or girls on a bus dedicated to Bachurim. Certainly it does not make sense to do this on a regular basis.
This is something the Hanhalah should determine.
.ראה נו״כ השו״ע אה״ע כב, ה. קיצור שו״ע קנב, ג. וראה שיחת ש״פ נח תנש״א