That's the northern part of Praha-Smíchov station ('severní' means north in Czech). If you look up aerial photographs of 50°03'46.2"N 14°24'23.8"E you can see the little platforms as well as an additional station building.
Here's a video (not mine) of the railbus in action, with Praha hlavní nádraží at the beginning and Praha-Smíchov severní nástupiště at the end:
ČD 810 'regiomouse' / trip praha hl.n. Praha-smíchov sev.n. / schienenbus (s65) - YouTube
That whole area has now been totally blitzed, and is now a massive construction site. However, Smichov North was a totally separate station to the main Smichov station. I once made the mistake of trying to catch a Semmering line train from Smichov, only to find no mention of them in any timetable or departure boards or signage. Smichov North had no station buildings, or shelter of any kind, for passengers.
There is no direct connection between the main Smichov station and the "Semmering" line.
Semmering line trains used to start from Smichov na Knizeci station, which was an old goods shed loading platform accessed by an unsigned footpath from the far side of Andel bus station. The trains then worked their way through the abandoned goods yard to the north west of Smichov station. A halt was provided close to Smichov station, consisting of a small brick station masters house complete with rose garden and name board Smichov, but no platform or obvious means of public access. Trains would stop, the station master complete with red hat and a red flag under his arm would appear and solemnly salute the driver, and off we would go again.
When they sold off the goods yard and na Knizeci station for redevelopment, they built the Smichov Severni station in the middle of the nearby carriage sidings. Trains ceased to stop at the old Smichov station halt, and its nameboard disappeared.
Now that the carriage sidings have also gone for redevelopment, Semmering line trains leave from Smichov station itself, but have to do a double set back to access the Semmering line. The second setback is done at the site of the old halt. The old station house, complete with rose garden and red hatted station master is still there, now sitting rather incongruously in the middle of a massive construction site.
Most Semmering trains no longer start from hlavni nadrazi, most now start from Smichov. The river bridge has been reduced to single track working, due to the condition of the bridge, with services between the two stations either side of the river much reduced.