Guard the caravans

Guard the caravans is a repeatable side quest in Fallout.

Crimson Caravan
Regarded as crazy by many, the Crimson Caravan offers the greatest variety of caravan routes to work on: Junktown, Boneyard, Necropolis and the Brotherhood of Steel. Their encounters are also the hardest and require good weapons and armor to make it to the destination, not to mention, encounters are guaranteed. The pay is 600 caps each way.

They only leave twice a month, on the 3rd and 17th. To get on a route, the Vault Dweller should talk to Keri in their Downtown office, after getting the all-clear from her father, Demetre.

Far Go Traders
Commonly regarded as the nicest merchant company in the Hub, the Far Go Traders offer good pay (400 caps each way), have moderately difficult encounters (and a 75% chance to even have one), and work the routes to Junktown, Boneyard and the Brotherhood of Steel. They leave three times a month, on the 8th, 18th and 28th.

Talk to Rutger and then to the caravan driver in their building to hook up with a caravan.

Water Merchants
The biggest company in the Hub is also the least profitable, at least to guards. They only work two locations, Junktown and Adytum, and offer 200 caps each way. The Water Merchants' routes offer the least encounters (5 in 10 times), and they leave the most often (1st, 5th, 10th, 15th, 20th, and 25th).

The Vault Dweller needs to talk to the caravan driver, outside the Water Merchants' HQ.

Rewards
200, 400 or 600 bottle caps depending upon which company the Vault Dweller worked for.