Given the task definition, example input & output, solve the new input case.
In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.
Example: Passage: Nearing London, Oliver encounters Jack Dawkins, a pickpocket more commonly known by the nickname the "Artful Dodger", and his sidekick, a boy of a humorous nature named Charley Bates, but Oliver's innocent and trusting nature fails to see any dishonesty in their actions. The Dodger provides Oliver with a free meal and tells him of a gentleman in London who will "give him lodgings for nothing, and never ask for change". Grateful for the unexpected assistance, Oliver follows the Dodger to the "old gentleman's" residence. In this way Oliver unwittingly falls in with an infamous Jewish criminal known as Fagin, the gentleman of whom the Artful Dodger spoke. Ensnared, Oliver lives with Fagin and his gang of juvenile pickpockets in their lair at Saffron Hill for some time, unaware of their criminal occupations. He believes they make wallets and handkerchiefs.
Output: Who believes Fagin's gang make wallets and handkerchiefs?.
This question is based on the following sentence in the passage "He believes they make wallets and handkerchiefs". It evaluates the understanding that the pronoun "he" refers to name "Oliver". You can ask questions like this one about most pronouns in a paragraph.

New input case for you: Passage: As the 19th century progressed, the United States government's Indian removal policy pushed Native American tribes west of the Mississippi River.  In 1830, the Indian Removal Act was signed into law, and though that act did not directly address the Potawatomi people of Indiana, it led to several additional treaties that resulted in their removal.  In what came to be known as the Potawatomi Trail of Death, about 860 Potawatomi Indians who had refused to leave were forced to move from Indiana to Kansas.  On September 14, 1838, the group camped near Williamsport, and on September 15 they camped in the southwestern part of the county before moving into Illinois.  Before reaching their destination in Kansas, over 40 of them had died, many of them children; two children died and were buried at the second Warren County campsite.When the county was established, the Wabash River was vital to transportation and shipping.  Zachariah Cicott traded up and down the river, and cities like Attica, Perrysville, Baltimore and Williamsport were founded near the river's banks and flourished because of it.  In the 1840s, the Wabash and Erie Canal began to operate and provided even broader shipping opportunities, but the canal favored towns which were on the "right side" of the river; the canal was on the Fountain County side, and towns like Baltimore dwindled as a result.  Some towns, such as Williamsport and Perrysville, managed to participate in canal traffic through the use of side-cuts that brought traffic from the canal across the river.  When railroads were constructed starting in the 1850s, they in turn began to render the canals obsolete and allowed trade to reach towns that lacked water connections.  The canal continued to be used through the early 1870s.
Output:
What decade did the canals that provided broader shipping opportunities start to decline due to railroads?