If Michigan commuters sweat less than us North East types, ok. But the reason for using vinyl for the seats has nothing to do with civility. Amtrak cleans its trains frequently, and exists for providing luxury, such that it is. Therefore, Amtrak is willing to take the expense of constantly replacing seat covers.
Commuter railroads are utilitarian things, not cleaned very often, and the expense of maintaining seats to luxury levels make the politico imbeciles start talking about "money wasting" and "pork". Vinyl lasts longer, and stays useable much longer. How long have the Arrow IIIs on NJT had those ugly brown covers without much replacement? 30 years?
As for the diaphragms, there are three kinds of connecting corridor types used on rail cars in this country.
Various subways, as well as twinned MUs on many eastern railways, have open corridors.
They are completely exposed to the elements, and do not wear out from heavy switching. Perfectly fine on the trains they are used, which have no passenger traffic between those cars, and no reason to have it since they either don't have restrooms at all (Septa, subways), or those restrooms set in the pairs (MNR, LIRR) and thus people use the covered route to the bathroom.
Then there are bumper diaphragms, used on all commuter railroads currently in service. They consist of rubber tubes. They don't wear out much from heavy switching, and provide a modicum of protection from the elements and falling off the train. They allow the occasional passenger to go wandering in search of a bathroom, or to move to certain cars when the entire train doesn't platform at their stations.
Then there are trifold diaphragms, which are large, complicated, and relatively delicate things. They provide almost complete protection from the elements. The downside is they wear out and require maintenance, and also require crew members to connect them manually from the inside. Amtrak uses them- the pass through of passengers between cars on a intercity train is pretty high. Most passengers will leave their normal car at some point during the trains journey. Like the cloth seats, they make sense for intercity trains, given their market and purpose.
On a commuter train, they border on the silly.