Passenger trains by their very nature are very different from airplanes, and are closer to other types of public environments like supermarkets. Trains are much less vulnerable to terrorist attacks from passengers onboard than airplanes are, so I'm not sure how much benefit there is from adding airport-type security for train passengers.
Airplanes are pressurized, have extremely lightweight structures, take a long time to descend, land, and evacuate from cruising altitude, and don't have cabin separation (except for the cockpit). They can only be landed by highly-trained pilots. Terrorist attempts to blow up airplanes have involved fairly small amounts of explosives (as shown by the thankfully unsuccessful shoe and underwear bombers). As unfortunately illustrated by 9/11, airplanes can also be weaponized as missiles and used to destroy skyscrapers and other large buildings and cause thousands of casualties. You therefore need fairly high levels of security for air travel.
Trains by comparison are heavy and overbuilt, sit on the ground, have doors between cars that allow on-board attackers to be compartmentalized, and can be brought to a stop in less than a minute and quickly evacuated. A shoe or underwear bomb will do little damage to a train. Emergency brake handles are located throughout the train and anyone can bring one to a stop, and they're difficult to weaponize as they only go where the track goes. A mass shooter can still do a lot of damage as passengers in the same car would have to escape and evacuate along a single aisle, but passengers are not that much more vulnerable than people at other public locations like supermarkets, shopping malls, transit buses, or say, train stations themselves.