After the birth of my second child, I got a job at a restaurant. Having worked with an experienced waitress for a few days, I was allowed to wait tables on my own. All went well that first week. When Saturday night came, I was luckily given the tables not far from the kitchen. However, I still felt a little hard to carry the heavy trays (托盘)。Before I knew it, the restaurant was full of people. I moved slowly, minding every step. I remember how happy I was when I saw the tray stand near the tables, it looked different from the one I was 【trained on】. It had nice handles (手柄)，which made it easier to move around. I was pleased with everything and began to believe I was a natural at this job."【】"里那个on是哪里来的啊？？？？
我看不明白啊

其实改成陈述句就是
I trained on the tray 我在架子上训练
train 在表示锻炼是是不及物动词，所以后面需要连接介词
train on 练好; 把...对准


train
n.
火车, 列车, 行列, 后果, 顺序, 长队, 长袍(裙)拖地的部分, 导火线

vt.
训练, 培养, 瞄准, (园艺)整枝

vi.
锻炼