Is this the 1891 robbery? A news article cites the RGRR annual report and another newspaper:
"The 1891 annual report of the railroad reported on the robbery of a train:
'On January 10, 1891 the train leaving Brownsville for Point Isabel was derailed about two miles from Brownsville by robbers. The engine and seven cars were wrecked and burnt. The engineer was badly scalded from the steam and one brakeman fell and had his leg and foot badly smashed by the wheels running over them. Both of these men are now doing very well.'
A 1983 article in the Valley Morning Star reviewed the history of this robbery, reporting that the train was carrying $40,000 of silver, to be shipped on a Morgan Line ship. The robbers had unbolted the bars that held the tracks, causing the train to derail. They robbed the passengers, locked them in the train and made off with some of the silver.
A railroad employee was waiting for the train to pass, wondered why it was late, and then saw smoke. He ran down the tracks and released the passengers. City Marshall S.A. Brito found a wrench at the scene that looked familiar, and based on his suspicions, arrested five men."
There seems to be some difference of reports of how much was taken. The Handbook of Texas Online article on Santiago Brito says $75,000 and speaks of a lawsuit against the railroad by a company that lost just under $20,000 in gold and silver.
In 1891, the Brownsville newspaper was called the Cosmopolitan. It became the Herald in 1892. If there are archives of the Cosmopolitan, there should be a story about the robbery and capture that might indicate movements. It caused a big stir over whether Brito, the ex-sheriff working for the railroad, or the current sheriff should hold Jose Mosqueda and the other prisoners that had to be settled by the U.S. Marshal taking over. Two defendants went to prison. I assume that others arrested went free. If they knew where the money was, they no doubt got it, or there was ample opportunity for them to get word to relatives. And the article in the Valley Morning Star should be findable.