How did they convert ac to dc current on electric train transformer when they didnt have transistors then?

Depends on whether you mean model trains or full-size trains.

Full-size first.

Streetcars and short-haul electric railways used DC motors powered from an overhead wire that was energized by rotary converters in enclosures spaced along the right of way. A rotary converter is a motor-generator in which an AC motor turns a DC generator.

Mainline electric railways most often used AC locomotives.

To simplify the issues of control and regenerative braking, engineers looked for ways to transmit high-voltage AC power through the overhead wire and run DC traction motors from the AC overhead. The 1950s solution was the Mercury Vapor Rectifier, or "ignitron."

Try an internet search on "mercury vapor rectifier."

There were many variations on these arrangements. Try an internet search on "electric railways."

Model trains.

Up until the 1940s, most model trains ran on AC. Classic Lionel trains were AC powered. DC was first used in HO railroading, and the most common source was a car battery, which is why the standard voltage is 12 volts.

Later, power packs became available using selenium rectifiers. The square plates were the heat sink, and the selenium rectifier was the cylindrical part in the middle. Try an internet search on "selenium rectifier."

Vacuum tube rectifiers are not suitable for model railroad use, and were never used in model railroad power packs.

By the way, a "transformer" in model railroad terms is always an AC device, as in Lionel. A "power pack" is a DC device.

Try an internet search on "Lionel train transformer."

Electric Train Transformer

You don't need a transistor. You just need a diode. Before they had solid state small diodes they had big chunky ones made of metal plates covered with selenium which had the same one-way-current properties you need to make DC from AC.

An ideal transformer is a equal voltage per turn device. If the secondary winding has 28/115 times the number of turns as the primary winding then it will produce 28/115 as much voltage as that applied to the primary. But an ideal transformer is also a power in equals power out device, so that the product of primary voltage times primary current equals the product of secondary voltage times secondary current. So the secondary current will be 115/28 times the primary current. -- Regards, John Popelish

Good questions so many answers. One technick is ther I forgot the name but to convert Ac in to DC in electrric train there is one motor coupled with DC generator. Motor Drives the DC generator and can get the out put.

Big electric trains, I mean -real- ones, ran on AC. For small electric trains, solid-state silicon diodes predated transistors by several years. And before that they used vacuum tube rectifiers.

Before there were solid-state silicon diodes (which is what you use to convert AC to DC), there were diode tubes for high-current rectification, and selenium rectifiers for low.

They used vacuum tube rectifiers.

The ones for large power applications were usually mercury vapor rectifiers. Some of these vacuum tubes were several feet tall, with a heated pool of mercury at the bottom and large electrodes inside. They glowed bright blue when they were in operation.

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They used metal rectifiers.

They would use a diode that would handle enough current.