I did it! (July 2019). Details and the map below.
Long story short, I needed an Immigration Office in Bangkok to extend my tourist visa. It is located in Chaeng Wattana Government Complex, a stadium-sized building that houses ministries, departments and other administrative agencies including Thailand Immigration Office. The place is situated in Lak Si District of Bangkok, in a remote area. The directions to the place can confuse. What many people do is hiring a taxi. Some public buses from Victory Monument that travel every 1 hour ...
As a result, I chose a railway trip from Hua Lamphong Train Station.
Since I had before traveled to Ayutthaya City and back by the same railway line, I experienced that there is no lack of trains in this direction. And you can check the train timetable between Hua Lamphong and Lak Si Station here:
Check and copy schedule since trains are often over there but they have a longish break in the middle of the day.
Still, when I was researching online for the trip, I felt unsure about the route between Lak Si Train Station and the Immigration Office. Is everything alright over there?...
So, I am going to share all necessary details below to assist people who have the same doubts.
Train trip from Hua Lamphong to Lak Si
As for me .. Once, I got a 7 a.m. train at Hua Lamphong. The timetable says the time of arrival at Lak Si is 7:41. But the train departed later, at 7:20. Trains often start late at Hua Lamphong and they go a bit slower then it is promised on the website... So we arrive at Lak Si Train Station at 8:11.
The price of the ticket for the trip is sometimes 20+ baht and sometimes 4 baht. It depends on the type of train - express and ordinary one (but they are just the same, honestly). Check the railway link above for prices.
At Lak Si Station
Lak Si Train Station is a small place beside giant concrete pillars of a toll road. It has benches and sheds, a ticket office and a motorbike taxi stand.
The stand is located closer to the bridge... You will easily find it.
They have a poster with official prices. A trip from Lak Si to Immigration Office costs 40 baht.
I visited the Office twice so I traveled by bike taxi two times both ways as well as I walked along this route and, once, I even had a bus from the main entrance of the Government Complex to Victory Monument (waited for 1 hour and a long trip)...
Walk from Lak Si to Thailand Immigration Office
How to walk from the train station to Chaeng Wattana Government Complex and, accordingly, Thailand Immigration Office located within it.
I will show on the map:
This is how a motorcycle taxi drove me when I arrived here for the first time and this is how I walked when I came to the place again.
The distance is exactly 2.3 km. So this is not far at all.
I walked for 35 minutes and it was so long because I was taking photos and bought and drank cappuccino in a 7/11 supermarket - yes, there is our favorite convenience store on the way! - 25 baht for a cup of hot natural coffee - they have a cooking stall in the shop. Good! Great!
One more fear bout the remote Thai roads and sois - stray dogs... Are there any between Lak Si and Immigration?...
But it was fine, no threats, no dogs alert from my side - the route is safe! Of course, this is Thailand and you never know but, generally, this route looks not a place for dog herds.
Extra advise for those who go to Immigration Office
Some extra information ... At the Chaengwattana Government Complex, they have this nice food and clothing market just inside of the building so you will have a place to eat over there with normal Thai prices like 40 baht or 50. I loved South Thai food I had there just 40 baht for a good portion! Enjoy!
Come early to the place, in the very beginning - it is overcrowded (in July!).
If you are traveling through Thailand and you want to extend your tourist visa just somewhere... Don't do it in Bangkok!! Any other Immigration Office in Thailand, not here! Too many people and you are not a traveler over there, not a guest, you are a chip in the ocean of Thai bureaucracy over there! Things you can do for 10 minutes in Immigration in Krabi or Prachuap Khiri Khan will take all your day in Bangkok...