the person who loses at "rock, paper, scissors" has to line up behind the winner to form a "train"
「じゃんけんで負けた人は勝った人の背後について」＝ the person who loses at "rock, paper, scissors" has to line up behind the winner to form a "train"
英語でルールを説明する際にこういう文章を使えば良いと思います：
Let's play "janken train".
（じゃんけん列車をしましょう。）
Make pairs and do "rock, paper, scissors”
（ペアになってじゃんけんをして。）
The loser lines up behind the winner and puts their hands on his or her shoulders.
（負けた人は勝った人の背後につき、列車のようになります。）
The winner finds another partner and plays janken again.
（勝った人はもう一人のパートナーを探してもう一度じゃんけんをします。）
The train gets longer and longer.
（じゃんけん列車はどんどん長くなります。）
You play rock, paper scissiors and then the loser goes behind the winner each time to form a line which gradually gets longer as winners are eliminated.
Hopefully this explanation is what you mean - so there are pairs of children and they play each other, the losers forming a line behind the winners, who play each other again. Ulitimately, all the children will be left in a row with the winner at the front of the line and no further contestants to play. But of course, the game may then be repeated.
When playing rock paper scissors, the loser lines up behind the winner with his hands on the winners shoulders to form a train.
▪ When playing rock paper scissors, the loser lines up behind the winner with his hands on the winners shoulders to form a train. The next playing will challenge the winner and the loser will join the line behind the winner making the train longer. And then the steps repeat until there are no more challengers.
The winner will always be in front and all the losers will be behind the winner in a line like a train.
・ When playing rock paper scissors, the loser lines up behind the winner with his hands on the winners shoulders to form a train.
The next playing will challenge the winner and the loser will join the line behind the winner making the train longer.
And then the steps repeat until there are no more challengers.
The winner will always be in front and all the losers will be behind the winner in a line like a train.
上記のような説明でどうでしょうか？
意味合いとしては：
(じゃんけんをして負けた人が勝った人の後ろに付き、勝者の方に手を置き列車のようになります。
次の子が勝った人とじゃんけんをして負けた人が勝った人の後ろに付き列車が長くなっていく。
挑戦者が居なくなるまでこれを繰り返します。
じゃんけんに負けた人は勝った人の後ろにつき一列につながった列車のようになります。)
When playing "rock, paper, scissors" the loser lines up behind the winner to make a 'train'
By saying 'when playing' it explains what you are doing (playing rock, paper, scissors)
The loser suggests the person/player who does not win, behind means to stand behind so by saying 'behind the winner' means to stand after the winner (behind them)
To 'make a train' meas a line of people
So you would say 'When playing rock, paper, scissors, the loser lines up behind the winner to make a train'
「when playing」は、そのときしていること（じゃんけん）を表します。
「The loser」は負けた人のことです。
「behind」は、後ろにつくという意味です。「behind the winner」で、勝った人の後ろにつく、となります。
「make a train」は、人の列を表します。
したがって以下のように言えます。
When playing rock, paper, scissors, the loser lines up behind the winner to make a train.
（じゃんけんをして、負けた人は勝った人の後ろについて列を作ります。）
We are going to play rock, paper, scissors in pairs. The winner will play with the next person but the one who loses will have to stand behind the winner with their hands on their shoulders. So, we are going to be forming a train
When you want to explain game instructions, it's always available to shorten the various aspects so that you're clear. Firstly, you can start explaining what game you are going to play and that it's going to be played in pairs. Then you can explain that each child that loses will have to stand behind the one who wins with their hands on the winner's shoulders. Then explain that the winner will keep playing with the next person until a train is formed. For example, you can say:
-We are going to play rock, paper, scissors in pairs. The winner will play with the next person but the one who loses will have to stand behind the winner with their hands on their shoulders. So, we are going to be forming a train
ゲームのやり方を説明するときは、伝わりやすいように、省略できるところは省略して構いません。
まず最初に、何のゲームをするのか（じゃんけん）、またそれをペアですると伝えましょう。次に、負けた子どもは勝った子の肩に手を載せて、その子の後ろに立つよう言います。最後に、勝った子は電車ができるまで、じゃんけんを続けると説明します。
例文
-We are going to play rock, paper, scissors in pairs. The winner will play with the next person but the one who loses will have to stand behind the winner with their hands on their shoulders. So, we are going to be forming a train
（ペアでじゃんけんをします。勝った人は次の人とじゃんけんをしますが、負けた人は手を勝った人の肩においてその人の後ろに立ちます。このようにして電車を作ります。）
When playing rock, paper, scissors the one who loses gets behind the winner and by repeating this they form a train.
The game is called Janken train. It consists of two parts.
First the players compete in a game of rock, paper, scissors.
Then the loser lines up behind the winner and puts their hands on his or her shoulders. This way players will form a train.
The game is called Janken train. It consists of two parts.
First the players compete in a game of rock, paper, scissors.
Then the loser lines up behind the winner and puts their hands on his or her shoulders. This way players will form a train.
↓
〈訳〉
このゲームは「じゃんけん列車」といいます。これは二つの部分により構成されます。
まず、参加者はじゃんけんをします。負けた人は勝った人の後ろに付き、手を前の人の肩に載せます。これによって参加者は列車を作ります。
The loser of each game of rock, paper, scissors has to line up behind the winner of the game.
The loser of each game of rock, paper, scissors has to line up behind the winner of the game.
Children line up in pairs and play the game of rock, paper, scissors. At the end of each game, the losing child must line up behind the winner of the game. This continues until there is a single "train" (line) of children with the winner at the start of the line.
The loser of each game of rock, paper, scissors has to line up behind the winner of the game.
［訳］じゃんけんで負けた人は、勝った人の後ろに並ばなければなりません
Children line up in pairs and play the game of rock, paper, scissors. At the end of each game, the losing child must line up behind the winner of the game. This continues until there is a single "train" (line) of children with the winner at the start of the line.
［訳］子どもたちがペアを組んで並んで、じゃんけんをします。じゃんけんで負けた子は勝った子の後ろに並びます。1本の列（電車）ができるまでこれを続けます。
イギリス 
南アフリカ 
カナダ 
