trailers placed between two flat cars

My father and I go train watching in North Bergen Yard, NJ. There CSX has an intermodal facility. I have seen many intermodals in my days but something always made me wonder why trailers are placed between two flat cars. Can anybody give me an answer on why railroads do this?

You mean accross two flat cars. cause there is no problem doing so.
The trailer is only secured at hitch structure so the wheels can move back and forth with no problem.
The flat cars are 87 feet long , the trailers are 53 feet for longest I believe, so two 87 feet permanent coupled flatcars can accomodate 3 trailers.

As discussed in the post below it's a capacity issue. The practice permits two 87 foot flats carry a total of three 53 ft trailers.

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A couple clarifications:

1 - Trailers are loaded only between pairs of semi-permanently coupled flats (cars joined by a solid drawbar instead of the usual knuckle coupler as in this image). Consequently no slack action occurs between the paired cars.

2 - Modern trailers all have spring-apply/air-release parking brakes. Thus trailer's road wheels are always locked except when attached to a road tractor ard air applied to the parking brake release line. Were slack action to occur between cars it would scrub the tires each time the slack ran in/out.

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Please note that the equipment picture above is connected by a solid
drawbar which makes it one car. This is common, I see cars of this type
here in South Florida very often.
Noel Weaver

Reporting marks "TTEX" if I remember correctly.

is your standard road trailer able to be toplifted onto a Flatcar , or are they special trailers ?

The trailers carried on these flatcars or other intermodal carriers are reinforced to allow them to be lifted. The majority of highway transport vehicles are not able to withstand such treatment.

Canadian Pacific's Expressway (based on the older "iron highway" originated with CSX) has drive-on/drive-off platforms which allow almost any standard trailer to be carried.