They are largely ornamental and cannot really take much of a bump. Although a friend informed me that he rode four empties into one and it stopped them right now. They might more properly be called, tapping posts. All the arrival tracks in Chicago Union Station end at a bumping post.
During the course of my career as a conductor working out of Chicago I had ample time to become familiar with them.
There are only two ways a train can arrive in the station, engine first (pulling in) or backing in. When a train pulls into the station it is the engineer's responsibility not to hit the bumping post. If the train backs in the station that responsibility falls to the conductor.
It is the process of backing into the station that we would like to address.

When backing in, the conductor stands in the rear of the last car of the train, operating the train's brakes through the use of a device called a tail hose. In later years the train's movement while backing up was controlled by voice command to the engineer via radio. But prior to that (and in the event of radio failure) all communication between the head end and the rear end was done using the tail hose.
By opening the valve on the tail hose and drawing air out of the brake system, the brakes apply. Closing the valve restores the air pressure and releases the brakes.
In Chicago, the bumping posts are attached to large reinforced concrete pylons. These are constructed in such a way as to withstand a pretty good bump. If a train moving at walking speed strikes the bumping post it will come to an abrupt stop, usually derailing a few cars. Of course, this is to be avoided at all costs since the passengers are out of their seats and moving about during these last moments before arrival.
To make an emergency brake application using the tail hose one merely opens the valve rapidly to full exhaust. This is called, dumping the air. A train moving at walking speed will begin to slide with the wheels locked if the air is dumped. And standing passengers will fall over.
A train hitting the bumping post at anything more than walking speed will knock everything right out of the ground, reinforced concrete and all.
Near the end of my time backing trains into the station the fixed bumping posts were replaced by fancy pneumatic ones. A long piston/plunger type affair was fastened onto the concrete pylons. The idea was to prevent the sudden jar in the event of a train actually hitting the post. These did work to a certain extent, at very slow speeds, below even walking speed.
It was about this time that two or three trains in as many days hit the bumping post while backing into the station. There was consternation amongst the powers that be, and it was decided that rather than the road crews backing their own trains into the station, a special two-man crew consisting of a conductor and an engineer would back each and every train. Engineer Milt Fritz and I were relegated to these duties. I was flattered to be given this responsibility.
By and large, backing in trains all day went relatively smoothly. But not always. The radio airwaves around the station are very crowded and often there were several conversations going on at once while we were trying to make our final stop at the post. If more than one person talks on a radio frequency at the same time neither of them can be heard. They sort of cancel each other out. One tries not to key up (transmit) if the channel is in use. It was necessary to keep one hand on the tail hose brake valve at all times.
On one particular occasion as I was giving the remaining distance to our final stop to Milt, several other radio conversations began at the same time. Since we didn't seem to be slowing down I keyed up again, saying, Easy, Milt, three car lengths. For reasons unknown, at that moment Milt was actually notching out the throttle a bit and we speeded up.
We were shoving into track 24 or 26, where the union station yard master has his 'glass house' office situated behind the bumping post. I could clearly see Oscar' s (Jimmy Hederman) face as he looked up to see if we were really going to stop. I was beginning to wonder the same thing. It suddenly dawned on me that Milt was not hearing a word I was saying. Since we had speeded up there was no time for gradual stopping of the train and I was forced to dump the air. We began to slide.
As I braced for the impact I noticed Oscar evacuating his seat behind the desk in anticipation of bolting for the door. The wheels finally began to grip the rail and we decelerated somewhat as we made contact with that pneumatic plunger. In an instant the plunger was compressed its full 12 feet of travel. Fortunately, that's about when the train ran out of momentum and came to a stop, holding the plunger fully compressed. Oscar and I looked at each other, both shaking our heads. That was close.