As another answer mentions, but including here to make this complete: benefits in kind are taxed as part of income unless there's a special exemption.
Note that if you are getting a repayable loan you don't have to pay any tax at all, even on the fact that the loan is interest-free. There is a special exemption for interest-free loans up to £10k from your employer to buy a season ticket. It's only if they are handing you the season ticket outright that it's taxable.
If you are getting it paid for, then all income between approximately £43k and £150k is subject to direct tax at 40%, so that's what you would pay. However, you should generally be exempt from the 2% national insurance employees have to pay on normal income. Your employer will still have to pay employer's national insurance, but that's not your direct concern.
In addition, if you or your partner gets child benefit and you are the higher earner, you will have to repay some of that. At the precise numbers you mention (£50K income + £2K extra), the £2K will lead to you having to repay 20% of the total child benefit payments.
This extra "tax" applies only to incomes of between £50k and £60k where you are the higher earner: at £50k you get all the child benefit, at £60k you get none (or have to repay it all in your tax return), and in-between you repay some of it on a taper.
Child benefit is approximately £1k for the first child and £650 for subsequent children, making the cost to you approximately £200 for the first child and £130 per subsequent child.
Another way of looking at that taper is that for income between £50k and £60k, you pay 10% extra tax if you have one child, 16.5% if you have two children, etc.
Particularly if the child benefit withdrawal affects you, do make sure to take advantage of everything that can reduce your taxable income. For example, your employer may offer childcare vouchers tax-free. If you make donations to charity via gift aid, private pension contributions, or anything else that is deductible from tax, make sure to declare them on your tax return.