Chisígo, Aug. 1.- The case of the Detroit board of trade and the llerchants' and Manufacturera' association against the New York Central, Grand Truuk, and West Sbore railways, forming the through trunk line route, was begun before the inter -state eommigsion yesterday at the Palmer house. All the members of the commission were present. The testimony in the case is mostly in type-writing form, taken before a master ia chancery, and is very voluminous. The cumplaint is that for a long time the merchante and business raeu of Detroit have suffeted by unjust discrimination at the baud.s of this trunk line systetn in favor cf Chicago. In specifying the particulars oomplainants allege that the railroads mentioned form a through trunk line from Chicago to New York and the east, and that their profits on that class of business is divtded proportionately among them. Of tbe through rate, the Detroit people olaim that their proportion should not exceed 70 per cent. under the long-aud-short-haul clause, while in reality the railroad charged tliem TS per cent. Also that the ' 70 per cent. rate was granted to pointe west of Detroit, and in fact to all points exoept Detroit. This diverts business around Detroit. Respondents admit that they form a through line, but say that they are running undor a schud ule of rates fixed March 13 and in torce April 1, and not under the schedula shown by complainant. They also claim that the 78 per cent, rate to Detroit is just and reasonable, and that under the joint trafile arrangement between defendants and conuectioLS west of Detroit regarding the rate to and from Chicago and New York via Detroit, the defendaut's proportion is substantially 70 per cent. Other admissions are also made, so that tbe issue is squarely joined, and the case is one of law, the facts being substantially agreed upon. Tbe railroad companies claim and make as the basis of their defense the doctrine that dis tanca alone is not the only consideration that may influence the priceof transportation. Billing, weighing, Jnspeetiug, switching, etc., etc., should be cousidered. The testiraony given by the Detroit witnesses went to show that the rate had a disastrous effect on the city's business, while that for the railways proved that the terminal charges at Detroit were greater than at Chioago. Denison B. Smith and C. A. King, of the Toledo Produce Exchange, are in the city for the purpose of filing similar complaints against the same roads on behalf of Toledo.