She glances up and down the platform, absorbing data at lightning speed: the ambient temperature, the size of the passengers’ baggage, the books and newspapers tucked under their arms, what the youngsters are wearing and a bewildering range of other variables.

Behind her smile, a thousand calculations whirr frenetically in Kumiko Mogi’s head as she sizes up today’s market. “Salarymen on company visits, no kids, and it’s 35 degrees by 9am. Banana cakes and iced coffee are going to fly today,” she mutters to herself. “Beef and rice lunchboxes will do nicely after the Fukushima stop.”

She watches as the passengers board, many of them sweating. “Let’s get them their ice-cream,” she chirps, punching the air with determination.

At 27 she is, by an improbably vast margin, the most successful snack saleswoman on the Japanese bullet network — trains that move at 300km/h (190mph) but have no dining cars. The rank-and-file wagon girls average sales of about 100,000 yen (£450) over the two legs of a six-hour return journey. Mogi, a former champion at Shogi Japanese chess, collects more than five times that.

In the nine years since she left school to become a wagon girl, Mogi has surpassed all her colleagues and has been promoted to chief instructor of the 1,300 women who serve the East Japan Railways bullet trains. “I knew from the start that I would be very good at this job,” she says.

Good looks, she tells The Times sharply as she shares the secret of her success, are not important: it is these 30 seconds of detailed “people reading” on the platform that give her the edge. Despite the rigid controls that restrain individual flair throughout most mainstream Japanese retail, the ultra-competitive working life of the wagon girls is salesmanship at its most raw. Each is in complete and autonomous control of her small trolley — and of what more than 800 passengers will be able to snack on. There are no seats provided for the wagon girls; their job is simply to keep selling until the doors open at the terminus.

Each morning, before the bullet trains speed off to cross Japan, the women load the trolley from a stockroom at Tokyo station that contains more than 150 different items. When filled — and arranged in the most eye-catching way — the trolleys trundle through the aisles laden with about Y200,000 of beer, dried fish, hot drinks, salted beef tongue and ready-to-eat lunchboxes.

The potential damage of a poor choice is substantial: too many packets of dried scallops on a train with a lot of children makes a devastating dent in the profits that could be made instead on Pocky chocolate straws. Mogi’s success boils down, she explains, to being at all times the embodiment of the Japanese word genki — a combination of spirit, vigour and cheerfulness.

She works constantly to read and engage with the customers. She switches effortlessly between regional accents to suit the person in front of her. Sales staff across Japan are obliged by their companies to use the most polite form of speech; Mogi’s art is knowing when to turn it off. In a break with the constricted norms of Japanese conversation, Mogi talks expansively with her hands. “I do try to put a lot more emotion into my words than I would outside the train,” she admits.

But her greatest innovation was to pull, rather than push, the trolley through the train. “By pulling the trolley, and never turning my back towards the customers, I can constantly look at their faces and work out what they are going to want,” she says.

Many regard Mogi as a prime example of Japan’s failure to promote female talent. She is baffled at the suggestion that she might seek bigger opportunities. “But you’ve been watching me today,” she says. “You can see that I’m perfect for this job.”

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