What is the hidden purpose of the subway system which Jurgis helps to construct?

From the text:

The City Council had passed a quiet and innocent little bill allowing a company to construct telephone conduits under the city streets; and upon the strength of this, a great corporation had proceeded to tunnel all Chicago with a system of railway freight-subways. In the city there was a combination of employers, representing hundreds of millions of capital, and formed for the purpose of crushing the labor unions. The chief union which troubled it was the teamsters'; and when these freight tunnels were completed, connecting all the big factories and stores with the railroad depots, they would have the teamsters' union by the throat. Now and then there were rumors and murmurs in the Board of Aldermen, and once there was a committee to investigate--but each time another small fortune was paid over, and the rumors died away; until at last the city woke up with a start to find the work completed. There was a tremendous scandal, of course; it was found that the city records had been falsified and other crimes committed, and some of Chicago's big capitalists got into jail--figuratively speaking. The aldermen declared that they had had no idea of it all, in spite of the fact that the main entrance to the work had been in the rear of the saloon of one of them.