How can I make a typescript fixture that allows partial defaults to be supplied?

Question

Say I have a helper function to create text data in my fixture. The test data has a type definition of:

type Locomotive = {
  name: string;
  serialNumber: string;
  engine: Engine;
  model: Model;
  gauge: number;
};

type Engine = {
  fuel:  'COAL' | 'DIESEL' | 'ELECTRIC';
  size: number;
}

enum Model {
  BOMBADIER = 'Bombardier Transportation',
  FFESTINOG = 'Ffestiniog Railway',
  HORNBY = 'Hornby'
}

And my fixture generating function, using https://fakerjs.dev is:

import { faker } from '@faker-js/faker';

const getTrains = (count: number, defaultValues?: MockLocomotive): Locomotive[] =>
[...Array(count).keys()].map(index => {
  const { engine, ...otherDefaults } = defaultValues ?? {};
  return {
    name: faker.person.fullName(),
    serialNumber: `train${index}`,
    engine: {
      fuel: faker.helpers.arrayElement(['COAL', 'DIESEL', 'ELECTRIC']),
      size: faker.number.int({ min: 10, max: 99 }),
      ...engine,
    },
    model: faker.helpers.enumValue(Model),
    gauge: faker.number.int({ min: 100, max: 120 }),
  ...otherDefaults
  };
});

console.log(getTrains(2, {engine: {fuel: 'COAL'}}))

I've tried making the type of defaultValues:

interface MockLocomotive extends Omit<Partial<Locomotive>, 'engine | name'> {
 engine?: Partial<Engine>,
 name?: string,
}

When I try to debug using this playground setup, I get this error on MockLocomotive:

Interface 'MockLocomotive' incorrectly extends interface 'Omit<Partial<Locomotive>, "engine | name">'.
  Types of property 'engine' are incompatible.
    Type 'Partial<Engine> | undefined' is not assignable to type 'Engine | undefined'.
      Type 'Partial<Engine>' is not assignable to type 'Engine'.
        Types of property 'fuel' are incompatible.
          Type '"COAL" | "DIESEL" | "ELECTRIC" | undefined' is not assignable to type '"COAL" | "DIESEL" | "ELECTRIC"'.
            Type 'undefined' is not assignable to type '"COAL" | "DIESEL" | "ELECTRIC"'.

I don't know if this is exactly the same issue as in my real scenario, but it seems close enough that someone might be able to help. Of note, is the fact that if I don't omit 'name' I don't get the error...