Thank you. I had not read this before my trip, but this is what I ended up doing. I took the free shuttle bus to the Transportation Center and bus 553 from there.

The airport line is bout 100-150 meters from the Hua Mak station. The Hua Mak station was very close to the bus stop where the bus 553 dropped me. What I could see from the train as it went by the SRT Lat Krabang station, it might not be easy to get to it from the Lat Krabang station of the airport line.
After clearing customs and immigration, I had some coffee, organised my back pack for local travel, and bought a SIM card. It was 2:00 a.m. by then.on, I had some coffee, organised my back pack for local travel, bought a SIM card and got something to eat. It was 2:00 a.m. by then.

The information desk at the airport and the transportation are just about useless. When I asked how to get to Lat Krabang Station, the suggestion was "taxi-meter". When I asked about bus, they drew a blank. I explained I wanted to take the train to Aranya Prathet, the suggestion was "You take taxi meter to Hua Lamphong." I explained further that the train goes by the Lat Krabang station, she asked
"Where you go?" Aranya Prathet was my answer, which was met with "You go Moh Chit bas station" I explained again, "I want to take the train. The train from Hua Lamphong goes via and stops at Lat Krabang Station, which is very close to the airport. Is there a bus from the airport to the Lat Krabang station." It seemed like I got through this time. "Where you go Lat Krabang. Which street you go? If you give address maybe we can tell you which bas go there"
I decided to explore another possibility. I asked if there was a way to get to lat Krabang station from the transportation center. "Yes, you take taxi-meter". As for bus, I was told "It is too night now. Maybe you take public van in morning" Morning was Ok with me as long as it was before 6:00 a.m.

On the free shuttle to the transportation center, I talked to a lady who was headed home after the end of her shift at the airport. She said she was going to help me. When the shuttle bus stopped at the transportation center, her bus was ready to leave. She jumped off from the shuttle bus, waited for me to get off, talked to a guy pointing towards me and handed me over to him. He took me to an information desk. The information desk knew of no bus that could take me to Lat Krabang, and suggested "taxi-meter". Not only that, he claimed that the train arrived at Lat Krabang at 6:15, which is unlikley if it departs Hua Lamphong at 5:55 a.m. He also indicated there was a train every hour or so, which is true. But not all trains, that go by Lat Krabang, go to Aranya Prathet. The mere mention of Aranya Prathet prompted him to suggest that I take a bus from there (the transportation center). His advice, whose soundness I doubted, was to take a taxi. I decided to relax a little at the transportation center before deciding what to do. In Thailand, taxi is always an affordable solution for getting around.. It was now about 3:00 a.m. I went to the Seven Eleven there, bought a coke and a cup of instant noodles. The cashier was quite friendly and taught me a few Thai phrases. I did not want to get to the Lat Krabang station right then as there might not be any place there to buy snacks/water etc, not to mention if it would be safe there, even though the man at the help desk suggested I leave by 4:30. The transportation center was well lit with Seven Eleven. What else could I ask for? I walked around looking at signs for all the bus routes there. Two of them included the word Lat Krabang. Looking at the names of places en-route, my guess was that bus 553 should go to Lat Krabang station. I wasn't 100% confident with my guess. By about 4:00, other counters were beginning to come to life, including a BMTA counter. I enquired at the BMTA counter. The man confirmed my guess. I still was not confident as I have often got wrong information from people as they pick words and phrases they recognise while the rest of sentence flies over them. I don;t blame them. But I expected better communication skills at the airport information desk. At the European airports, including in Moscow, Japanese and some other Asian airports I have been through,I have found that more people I need to speak to speak better English than at BKK.
Bus 553 pulled up. Some young guys were standing near the spot where Bus 553 departs from. I asked one of them. Luckily, his English was significantly better than of those at the help desk. He confirmed that the bus did indeed go to Lat Krabang, but that Hua Mak was better and that he would tell me where to get off. He could not explain why Hua Mak was better. But he did say "because I am going there". I think the bus takes a circuitous route and gets to Lat Krabang after his stop and he wouldn't be on the bus to guide me. The bus fare was 30 Baht. He told me that the train station was only a couple of blocks from the bus stop. It was still dark (about 4:30) when I got off the bus. A man had come to drop is daughter off at the bus stop. I asked him "Train station?" made gestures and sounds to express myself, but no luck. So I started walking in the same direction as the bus had disappeared. The first left, just 50 yards beyond the bus stop lead to an open area with a building that could be a train station. As I made the turn, it dawned on the man that I was looking for the train station. He beckoned me to follow him, which I did. Within minutes I saw reflections from the tracks. Sheer joy is how I can describe the feeling.

Summary: I am surprised how little the airport help desk knows about anything. Their suggestion in response to every query about going somewhere in the city is "taxi-meter". My feeling is that I don;t need to ask people if I want to take a taxi. On my prior trip to Cambodia also I had sought help at the desk about catching the bus to Aranya Prathet, that is supposed to depart from the transportation center near the airport. They had no idea. One said that the bus had already left, while another said that the bus no longer goes from there. The information desk is right in front of the exit from the customs. Meeters and greeters congregate right in front of the information desk making it very difficult to get to them. On my return to the airport, instructions from my hotel at Suvaran Bhumi were to ask at the information desk if I did not see an employee from the hotel near the customs exit. I had not informed the hotel of the time of my arrival. I enquired at the information desk. Her response was "Hotel over there" raising her arm with great offered and pointing towards the exit as if it took a lot of effort. I didn't know what the hotel employee looked like, or what he/she wore for uniform. The area she pointed to was fenced in. I did n ot know if I was allowed to go there. So I asked her again about how and where to find the hotel staff. She became very irate, almost yelling. I then asked a person who represented a limo company. He spoke excellent English and called the hotel lady, who was inside the fenced area. She escorted me inside.
Private operators speak much better English and are more courteous than the airport information desk. Maybe the difference is due to who your employer is.

Conclusion: I did some more research about how to get to a station near the airport on the Eastern line of SRT. The best cheap way to catch an SRT train on the Eastern line, to go to Aranya Prathet or Pattaya, if you don't mind roughing it, is to exit the airport on arrivals floor through door 5. Cross the first lane and wait for the free shuttle bus to the transportation center. From there, take public bus 517 to Hua Ta Khae station. I think, the fare is 15 Baht.