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 Vault Dweller's destination, not to mention, he is guaranteed to have one. The pay is 600 Hub bucks each way, making for a total of 1200 caps both ways.

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 Traders offer good pay (400 scripts 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 bucks each way. However, the Vault Dweller is least likely to get an encounter on the way (5 in 10 times) and they leave most often (1st, 5th, 10th, 15th, 20th and 25th).

The Vault Dweller needs to talk to the dwarf outside the Water Merchants' HQ.

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