Would you want tankers full of combustable fuels or dangerous chemicals zipping around at 300 km/h or faster? The faster a train goes the harder it is to stop and the more likely the accident will be catastrophic.
I don't know. How's it different with wagons full of squishy passengers?
If this is how it works in real life, and the mod's just following realism, I understand. But it's rather counter-intuitive to see maximum speed values which are strictly theoretical in most cases.
As far as I can recall (having contributed to the capacity/speed balancing of coaches and freight cars of this set), 2cc TrainsInNML is generally pretty close to real-life values because the vehicle stats are based on real-world examples. In real life, freight cars rated to run at high speeds (say, over 75 mph) are pretty rare because of the higher cost of more sophisticated bogies and brake systems that would be required to permit them to run at those speeds in most countries (e.g. electromagnetic track brakes). In the most cases, this is not economical, thus freight trains exceeding 75 mph are usually rare exceptions.
For example, Germany's fastest freight train (Parcel InterCity)
runs at 87 mph, but only when governed by the LZB system on high speed railways. It used to hit 100 mph with ES64U locomotives and cars rated for 100 mph with electropneumatic brakes, but this was not economical - consuming too many train paths due to the high speed difference to regular freight trains and requiring bespoke rolling stock, with only four suitable trains available nationwide. At 87 mph under LZB control, there is a wider range of vehicles, offering more operational flexibility - if I'm not mistaken, these cars do not need electropneumatic brakes if 87 mph are only reached under LZB control. Beyond all that, the even higher energy consumption (for little benefit depending on how long it takes to hit top speed before it's time to brake again) is a factor as well, of course. The fastest container flat wagons I am aware of today are the Transmashholding 13-6954 from Russia with a 2'2' axle layout to carry 36 tons
at 100 mph / 160 km/h and the similarly-sized but lighter Kockums Industrier Lgss-y055, a 2-axle flatcar carrying 27 tons of Swedish mail.
Both of these fast container wagons are fairly short (15 to 17 m), carrying only up to one 40' or 45' container, as opposed to the 80' and 68.5 tons you can put on a 26 m single-chassis Sggns(s) 80'
with a 2'2' axle layout at 75 mph / 120 km/h. With a split chassis for a 2'2'2' axle layout, you can even bring the payload to 107 tons at the same top speed and length (e.g. Sggrs(s) 80'
For the relative payload and capacity per length, the higher-speed wagons fall short compared to conventional 80' wagons - and the same goes for acquisition and maintenance costs, based on more sophisticated brakes and bogies, as well as more axles per container (the latter only applies to the Russian wagon). These high speed vehicles are often built for a specific purpose, e.g. fast transport of mail (Sweden) or the fast transport of goods over long distances (e.g. InterCargoExpress & new Russian flatcar for carrying Chinese cargo to Europe across Russia quickly), and this comes at a high cost. When DB launched its 160 km/h InterCargoExpress operations in 1991, the vehicle acquisition alone already cost them about 67% more than for conventional freight cars. The Russian design is fitted with bogies taken from a passenger coach with electropneumatic brakes - and that solution is most certainly not cheaper than regular freight bogies.