‘At least I’m not a charity case,’ Thea said, but I could tell from the way she said it that the two were friends, and this goading banter was part of their act. ‘Kate’s father is the art master,’ she told me. ‘So a free place for his daughter is all part of the deal.’

‘No chance of Thea qualifying for charity,’ Kate said. Silver spoon, she mouthed over the top of the teas, and winked. I tried not to smile.

She and Thea shared a look and I felt some wordless question and answer pass between them, and then Thea spoke.

‘What’s your name?’

‘Isa,’ I said.

‘Well, Isa. Why don’t you come and join me and Kate?’ She raised one eyebrow. ‘We’ve got a compartment just up the corridor.’

I took a deep breath and, with the feeling that I was about to step off a very high diving board, gave a short nod. As I picked up my case and followed Thea’s retreating back, I had no idea that that one simple action had changed my life forever.

IT’S STRANGE BEING back at Victoria. The Salten train is new, with open-plan carriages and automatic doors, not the old-fashioned slam-door thing we used to take to school, but the platform has hardly changed, and I realise that I have spent seventeen years unconsciously avoiding this place – avoiding everything associated with that time.

Balancing my takeaway coffee precariously in one hand, I heave Freya’s pram onto the train, dump my coffee on an empty table, and then there’s the same long struggling moment there always is, as I attempt to unclip the cot attachment – wrestling with clasps that won’t undo and catches that won’t let go. Thank God the train is quiet and the carriage almost empty, so I don’t have the usual hot embarrassment of people queueing in front or behind, or pushing past in the inadequate space. At last – just as the guard’s whistle sounds, and the train rocks and sighs and begins to heave out of the station – the final clip gives, and Freya’s cot jerks up, light in my hands. I stow her safely, still sleeping, opposite the table where I left my coffee.

I take my cup with me when I go back to sort out my bags. There are sharp images in my head – the train jerking, the hot coffee drenching Freya. I know it’s irrational – she’s on the other side of the aisle. But this is the person I’ve become since having her. All my fears – the ones that used to flit between dividing trains, and lift doors, and strange taxi drivers, and talking to people I didn’t know – all those anxieties have settled to roost on Freya.

At last we’re both comfortable, me with my book and my coffee, Freya asleep, with her blankie clutched to her cheek. Her face, in the bright June sunshine, is cherubic – her skin impossibly fine and clear – and I am flooded with a scalding drench of love for her, as painful and shocking as if that coffee had spilled across my heart. I sit, and for a moment I am nothing but her mother, and there is no one in the world except the two of us in this pool of sunshine and love.

And then I realise that my phone is buzzing.

Fatima Chaudhry says the screen. And my heart does a little jump.

I open it up, my fingers shaking.

I’m coming, it says. Driving down tonight when the kids are in bed. Will be with you 9/10ish.

So it’s begun. Nothing from Thea yet, but I know it will come. The spell has broken – the illusion that it’s just me and Freya, off on a seaside holiday for two. I remember why I am really here. I remember what we did.

I’m on the 12.05 from Victoria, I text back to the others. Pick me up from Salten, Kate?

No reply, but I know she won’t let me down.

I shut my eyes. I put my hand on Freya’s chest so I know she is there. And then I try to sleep.

I wake with a shock and a belting heart to the sound of crashing and shunting, and my first instinct is to reach out for Freya. For a minute I am not sure what has woken me but then I realise: the train is dividing, we are at Hampton’s Lee. Freya is squirming grumpily in her cot, she looks like she may settle if I’m lucky – but then there’s another shunt, more violent than the first set, and her eyes fly open in offended shock, her face crumpling in a sudden wail of annoyance and hunger.

‘Shh …’ I croon, scooping her up, warm and struggling from the cocoon of blankets and toys. ‘Shh … it’s OK, sweetie pie, it’s all right, my poppet. Nothing to worry about.’

She is dark-eyed and angry, bashing her cross little face against my chest as I get the buttons of my shirt undone and feel the by-now routine, yet always alien, rush of the milk letting down.

As she feeds, there is another bang and a crunch, and then a whistle blows, and we begin to move slowly out of the station, the platforms giving way to sidings, and then to houses, and then at last to fields and telegraph poles.

It is heart-stoppingly familiar. London, in all the years I’ve lived there, has been constantly changing. It’s like Freya, never the same from one day to the next. A shop opens here, a pub closes there. Buildings spring up – the Gherkin, the Shard – a supermarket sprawls across a piece of wasteland and apartment blocks seem to seed themselves like mushrooms, thrusting up from damp earth and broken concrete overnight.

But this line, this journey – it hasn’t changed at all.

There’s the burnt-out elm.

There’s the crumbling World War II pillbox.

There’s the rickety bridge, the train’s wheels sounding hollow above the void.

I shut my eyes, and I am back there in the compartment with Kate and Thea, laughing as they pull school skirts on over their jeans, button up shirts and ties over their summery vest tops. Thea was wearing stockings, I remember her rolling them up her impossibly long slender legs, and then reaching up beneath the regulation school skirt to fasten her suspenders. I remember the hot flush that stained my cheeks at the flash of her thigh, and looking away, out across the fields of autumn wheat, with my heart pounding as she laughed at my prudery.

‘You’d better hurry,’ Kate said lazily to Thea. She was dressed, and had packed her jeans and boots away in the case resting on the luggage rack. ‘We’ll be at Westridge soon, there’s always piles of beach-goers there, you don’t want to give a tourist a heart attack.’

Thea only stuck out her tongue, but she finished hooking her suspenders and smoothed down her skirt just as we pulled into Westridge station.

Sure enough, just as Kate had predicted, there was a scattering of tourists on the platform, and Thea let out a groan as the train drew to a halt. Our compartment door was level with a family of three beach-trippers, a mother, father and a little boy of about six with his bucket and spade in one hand, and a dripping choc ice in the other.

‘Room for three more?’ the father said jovially as he opened the door and they clambered in, slamming the door behind them. The little compartment felt suddenly very crowded.

‘I’m so sorry,’ Thea said, and she did sound sorry. ‘We’d love to have you, but my friend here,’ she indicated me, ‘she’s out on day release, and part of the terms of her probation is no contact with minors. The court judgement was very specific about that.’

The man blinked and his wife gave a nervous giggle. The boy wasn’t listening, he was busy picking bits of chocolate off his T-shirt.

‘It’s your child I’m thinking of,’ Thea said seriously. ‘Plus of course Ariadne really doesn’t want to go back to the young offenders’ institute.’

‘There’s an empty compartment next door,’ Kate said, and I could see she was trying to keep her face straight. She stood and slid open the door to the corridor. ‘I’m so sorry. We don’t want to inconvenience you, but I think it’s for the best, for everyone’s safety.’

The man shot us all a suspicious look, and then ushered his wife and little boy out into the corridor.

Thea burst into snorts of laughter as they left, barely waiting even until the compartment door had slid shut, but Kate was shaking her head.

‘You do not get a point for that,’ she said. Her face was twisted with suppressed laughter. ‘They didn’t believe you.’

‘Oh, come on!’ Thea took a cigarette out of a packet in her blazer pocket and lit it, taking a deep drag in defiance of the ‘No Smoking’ sign on the window. ‘They left, didn’t they?’