Sammie Welch, 23, was given the letter as she sat with her three-year-old son Rylan who was asleep on her lap.
The scribbled message, written on a piece of scrap paper, signed 'man on train at table with glasses and hat' and included a £5 note so Sammie could buy herself a drink.
It read: "Have a drink on me, you're a credit to your generation, polite and teaching the little boy good manners.
"PS I have a daughter your age, someone did the same for her once. Hope when she has children she is as good a mother as you."
Sammie was on a First Great Western train from Birmingham to Plymouth on Thursday when a man handed her the note as he got off in Bristol at around 7 or 8pm.
She was trying to keep Rylan entertained during the busy service and said she was simply playing with him as he ate his dinner.
She said: All I was doing was entertaining him really. He was eating his dinner, I had made him a packed lunch, and we were laughing at the fact he kept calling his grapes, strawberries.
"He was just being a happy, laughing little boy. We were playing together and I was trying to keep him occupied.
"He normally gets quite rowdy and bored on long trains so I was doing my best to entertain him as much as I could."
After his dinner, Rylan fell asleep, sprawled across his mother and another seat.
When another passenger came to sit down Sammie quickly moved Rylan on to her lap, insisting the man took the seat rather than stand.
Shortly after, the unknown admirer handed Sammie the note before disappearing into the rush to get off the train.
Sammie, from Plymouth, said: "Both me and the man next to me were a bit confused at first.
"Once I'd taken it we realised it couldn't have come out of my bag because I was sat next to the window. It was then we both looked at it, and we were so shocked.
"I feel really overwhelmed by it all, it was just so lovely to get it. I was so shocked at the fact he had put the money in it, I couldn't quite believe it.
"I've put the money in a savings account for Rylan now, I thought that was the best place for it.
"Money is quite short for people nowadays so for someone to give me that was amazing, and I want to put it to good use."
She said: "When he described himself at the end of the note as the table across from me I was trying to rack my brains to see if I could remember him but there were that many faces on the train they all blur into one.
"I really, really wish I could remember him but I just can't, I think he was around his early 50s, but I don't want to offend him if I really got it wrong.
"I'm trying to hard to recall the people and put things together but because he was getting off the train it all felt so rushed.
"I have no clue so I'm hoping people might be able to help me out.
"I wish I had looked at him in more detail, I really want to thank him personally."