The phrase light at the end of the tunnel denotes a long-awaited sign that a period of hardship or adversity is nearing an end.

This phrase occurs, for example, in a letter to the Editor from one Frederick Naerebout, Delft, the Netherlands, published in The Guardian (London and Manchester, England) of Saturday 29^th April 2023:

Emma Beddington writes about being embarrassed by the state of Britain when her French in-laws visited, citing various ways in which the country compares poorly with France (It’s shaming to suddenly see Britain through others’ eyes, 24 April). But she ends on a positive note, describing how her in-laws were impressed on a train when they saw “a lovely group of women sitting next to us in Hawaiian garlands, enjoying prosecco at 8am”. If that is the light at the end of the tunnel, it’s not much.

In light at the end of the tunnel, the image is of a railway tunnel, and the phrase has been used literally—for example in the following from The Manchester Courier, and Lancashire General Advertiser (Manchester, Lancashire, England) of Friday 8^th March 1867:

DREADFUL RAILWAY ACCIDENT,
NEAR BRADFORD.

Yesterday, a fearful accident occurred on the Lancashire and Yorkshire Railway, near Bradford, owing to one train running into another, in the Bowling tunnel. One man, Charles Goderich, an engine driver, was killed, and ten or twelve passengers were dreadfully mutilated. The train for Huddersfield left the Lancashire and Yorkshire Station at the usual time, 1 15 p.m., and when about half-way through the Bowling tunnel an accident happened to one the valves of the engine, which brought the train to a stand. At this moment a train from Leeds ran up, and coming in contact with the Huddersfield train, a dreadful collision took place. The engine of the Leeds train, and one or two of the carriages were smashed to atoms, and several of the carriages belonging to the other train. Great alarm and excitement was created, and for at least 20 minutes neither light nor signal, guard, or any of the officials could be seen or communicated with. All the while the piercing cries and wailings of the injured were heard. After the lapse of about half an hour the steam subsided, and the passengers began to be able to see the light at the end of the tunnel. They left the carriages and walked—some to the Low Moor Station, others to Bradford.

The earliest figurative uses of the phrase light at the end of the tunnel that I have found are as follows, in chronological order:

1-: From a letter by one J. Hooker, published in The Queen: The Lady’s Journal & Review (London, England) of Saturday 25^th October 1862—the reference to the French poet, novelist and playwright Victor Hugo (1802-1885) is obscure:

FEMALE EDUCATION.

DEAR MADAM,—It is a common axiom, but not the less a fallacy, that the fault-finder should provide a remedy; in which case all abuses might go on unchecked, and all social evils unsatirised, until the perfect cure is found out, and ready for application. If this were the right course to pursue in the subject before your readers, I should drop my pen in despair; for although I am quite satisfied that female education in the present day is grievously faulty, I do not see the light at the end of the tunnel, of which Mr. Victor Hugo spoke so touchingly, nay, I cannot be quite sure that I am on the right line of rail to arrive at the desired goal.

2-: From the North British Daily Mail (Glasgow, Lanarkshire, Scotland) of Friday 3^rd April 1863:

The Paisley petition […] begged a sum of money from the Imperial Treasury for the promotion of emigration from the manufacturing districts, and in the meantime a sum for the alleviation of immediate distress. Should the Legislature vote any money for the manufacturing districts, we hope that it will all be applied to the promotion of emigration. Of the money they have now in hand the Relief Committees might, without any breach of faith, devote a considerable portion to the same purpose. Were it to be determined that such was to be the application of any farther money that might be subscribed, both home and colonial contributors would begin to give again with their original liberality. Light at the end of the dark tunnel would then be descried. At present we do not know that the tunnel through which the Relief Fund just keep up steam enough to enable the long train of manufacturing distress to drag its slow length along at a snail’s pace may not be interminable.

3-: From the transcript of a speech that the Reverend D. W. Thomas delivered during the conference of the clergy and the principal of the laity of the Diocese of Bangor, held on Tuesday 12^th January 1869, published in The North Wales Chronicle, and Advertiser for the Principality (Bangor, Caernarfonshire, Wales) of Saturday 16^th January 1869:

Now, looking at the questions before them that day, he felt his confidence in the prospects of the church in Wales reviving. He had been in the habit of looking at things with a feeling akin to despair, but now there seemed to be hopeful signs of improvement, he saw light at the end of the tunnel.